North Korea is believed to have carried out its fifth and largest ever test of a nuclear device, say South Korean military officials.
It comes after an "artificial" magnitude 5.3 earthquake was detected close to its nuclear test site on Friday morning,
Previous artificial earthquakes in the same area and of that magnitude have all been nuclear tests.
There has been no comment from the North as yet.
In a statement carried by the Yonhap news agency, South Korea's Joint Chiefs of Staff said: "We estimate the North has carried out the biggest-ever test."
It said the yield was estimated to have been 10 kilotonnes, almost twice the power of its last test in January. At that time the North said it had successfully tested a hydrogen bomb, but many analysts cast doubt on that claim.
Japan's meteorological agency said seismic activity was "different than in a normal quake"
The US Geological Survey, which monitors global seismic activity, said the tremor had been detected in the Punggye-ri nuclear test site in the north-east and was caused by "an explosion".
It said it could "cannot determine what type of explosion it may be, whether nuclear or any other possible type".
Recent satellite imagery has shown increased activity at the site, indicating a fifth test could be imminent.
Friday is also North Korea's National Day, which celebrates the founding of the current regime. The North often uses such events as an opportunity for a show of military strength.
The death count from Ecuador's worst earthquake in a decade increased to at least 587 on Thursday even as the country faced another grim toll: a long and costly reconstruction effort likely to cost billions of dollars. On Thursday evening, the government raised the death toll to 587. Officials listed 155 people as missing and the number of those made homeless climbed to over 23,500. The final death toll could surpass casualties from earthquakes in Chile and Peru in the past decade. President Rafael Correa announced Wednesday night that he would raise sales taxes and put a one-time levy on millionaires to help pay for reconstruction. The damage from the 7.8-magnitude quake adds to already heavy economic hardships being felt in this OPEC nation because of the collapse in world oil prices. Even before the quake, Ecuador was bracing for a bout of austerity, with the International Monetary Fund forecasting the economy would shrink 4.5 percent this year. In a televised address Wednesday night, Correa warned the nation of a long and costly post-quake recovery and said the economic pain shouldn't fall only on hard-hit communities along the coast. “I know we're at the most-difficult stage right now but it's just the beginning,'' he said. Rescuers continued to comb through the rubble in coastal towns hit hardest by the quake, but the clock was running down for finding survivors. Rescue workers have said a person without serious injuries can survive up to a week buried in debris in the Ecuadorian heat. On Thursday, Jorge Zambrano, mayor of the community of Manta, announced that three people had been pulled from beneath a collapsed building the night before and vowed to continue searching. “I still have hope we can find more people who are still alive beneath the rubble,'' he said. “If we have even a hope of life, we must work with extreme care.'' Rescuers also found a more unusual survivor: a flapping white-and-brown duck was pulled from under a pile of rubble, becoming a social media celebrity in a country eager for good news. Using authority granted by the state of emergency he declared after Saturday night's quake, Correa said sales taxes would increase from 12 percent to 14 percent for the coming year. People with more than US$1 million in assets will be charged a one-time tax of 0.9 percent on their wealth, while workers earning over US$1,000 a month will be forced to contribute a day's wages and those earning US$5,000 a month the equivalent of five days' pay. Taxes on companies will also go up, and Correa said he will look to sell certain state assets that he didn't specify. He is also drawing on US$600 million in emergency credits from the World Bank and other multilateral lenders. The tax increases come as the scale of devastation continues to sink in. A helicopter flyover of the damage zone Wednesday showed entire city blocks in ruins as if they had been bombed.—AP
Saturday's magnitude-7.3 quake struck at 01:25 (15:25 GMT on Friday) close to the city of Kumamoto, which had been hit by a magnitude-6.4 quake on Thursday night.
Both quakes were shallow, causing huge damage to roads, bridges and tunnels. Big landslides cut off remote mountain villages.
The US military said it was preparing to provide aerial support for Japan's relief efforts. America has several military bases and about 50,000 troops in the country.
The earthquakes are the biggest to have hit Japan since 2011 when a 9.0 magnitude quake caused a huge tsunami, leaving more then 19,000 dead and missing.
Japan is regularly hit by earthquakes but strict building codes mean they usually cause minimal damage.
A state of emergency has been declared and some 10,000 troops and 3,500 police have been deployed in the affected areas, with an emphasis on finding survivors.
"I fear that figure will go up because we keep on removing rubble," a shaken Mr Correa said in a televised address.
"There are signs of life in the rubble, and that is being prioritised."
The magnitude-7.8 quake struck on Saturday evening. Coastal areas in the north-west were closest to the epicentre.
The quake is Ecuador's largest since 1979. More than 130 aftershocks have followed.
The US Geological Survey said the earthquake struck at a fairly shallow depth of 19.2km (11.9 miles), about 27km from Muisne in a sparsely populated area.
One million people had to leave their homes in Chile after a powerful quake hit the country's central region.
At least eight people died when the 8.3-magnitude quake hit. One person is still missing.
Residents of Illapel, near the quake's epicentre, fled into the streets in terror as their homes began to sway.
In the coastal town of Coquimbo, waves of 4.7m (15ft) hit the shore. A tsunami alert was issued for the entire Chilean coast but has since been lifted.
Tsunami waves also hit the coast further north and south of the quake's epicentre, with waves half a metre higher than usual as far north as La Punta.
The quake lasted for more than three minutes and there have been dozens of aftershocks.
Gloria Navarro, who lives in the coastal town of La Serena, said people were "running in all directions".
At the scene: Jane Chambers, freelance journalist
I'm on the coast about 130km (80 miles) south from the worst affected area of Coquimbo. Our house is on top of a cliff and made of wood. It was shaking and shuddering.
At first I thought it was just a tremor but it was really strong and went on for around three minutes. It was much stronger than any tremors I had ever felt before.
The house is fine as most of Chile's buildings are built to withstand tremors.
The local town was evacuated. The restaurant down on the beach is flooded but most things here are returning to normal.
Officials said 1,800 people in Illapel were left without drinking water.
Electricity providers said hundreds of thousands of their clients in the worst-affected Coquimbo region had no power.
Image copyrightAFPImage captionPeople fled into the streets as the quake caused buildings to swayImage copyrightAPImage captionThe city of Illapel was among the worst hitImage copyrightReutersImage captionThe quake also caused goods to fall from shelves in a supermarket in the city
The US Geological Survey (USGS) said the tremor struck off the coast of Coquimbo, 46km (29 miles) west of the city of Illapel at 19:54 local time (22:54 GMT).
The USGS said it was at a depth of 25km, while Chilean seismologists calculated its depth at 11km.
Analysis: Jonathan Amos, BBC Science Correspondent
Image copyrightReutersImage captionResidents of the coastal city of Valparaiso knew to take to higher ground
The quake that rocked Chile on Wednesday was five times more energetic than the one that devastated Nepal back in April. And yet the early indications are that the death toll will be a fraction (perhaps a thousandth) of what it was in the Himalayan nation.
In large part, this is simply down to preparedness. This was Chile's third massive quake in five years; the region all too frequently experiences magnitude 8 events. As a consequence, the building codes are strict and generally well enforced.
What is more, the people themselves are well versed in how to react during and after an event.
It is not perfect. In 2010, an 8.8-magnitude quake witnessed failings on the part of the monitoring network and the system for alerting people to the imminent tsunami threat.
Since then, the Chilean government has spent millions upgrading the country's seismic network of sensors, and made improvements to telecommunications systems that share critical information and warnings.
The earthquake struck as thousands of Chileans were travelling to the coast ahead of a week of celebrations for independence day.
President Michelle Bachelet said some of the official festivities would be cancelled.
The authorities were quick to issue tsunami alerts keen to avert a repeat of the slow response to the 8.8-magnitude quake in 2010, which devastated large areas of the country.
More than 500 people died in that quake and the tsunami it triggered and memories of the tragedy are still raw.
Image copyrightAFPImage captionChileans are terrified of a repeat of the deadly 2010 quake and subsequent tsunamiImage copyrightAFPImage captionMany spent the night in the street as aftershocks continuedImage copyrightAFPImage captionAs day broke, residents began to take stock of the damage
Tsunami alerts were issued shortly after the quake struck for the entire Chilean coast but have been gradually lifted, with the last cancelled at 06:22 local time.
President Bachelet said that "once again we must confront a powerful blow from nature". She will travel to the affected areas later on Thursday.
Chile is one of the most seismically active locations on the globe.
It runs along the boundary between the Nazca and South American tectonic plates. These are vast slabs of the Earth's surface that grind past each other at a rate of up to 80mm per year.
SANTIAGO, Chile - A magnitude 8.3 earthquake hit off the coast of Chile on Wednesday, shaking buildings in the capital city of Santiago and generating a tsunami warning for Chile and Peru.
Chile's government urged residents to evacuate the coastline. There were no immediate reports of injuries or damage.
Reuters witnesses said the quake was felt as far away as the Argentine capital, Buenos Aires, on the eastern seaboard of South America.
The quake struck 105 miles (169 km) north of Valparaiso and was originally reported as magnitude 7.9, the US Geological Survey said.
Hazardous tsunami waves from the quake were possible along the coasts of Chile and Peru within the next several hours, the Pacific Tsunami Warning Center said. A tsunami watch was also issued for Hawaii.
If tsunami waves impact Hawaii, the estimated earliest arrival time would be 3:06 a.m. Hawaii Standard Time (1306 GMT) on Thursday, the Pacific Tsunami Warning Center said.
The quake struck at a depth of 15.5 miles (25 km), the USGS said.
Less than an hour after the initial earthquake, three aftershocks all greater than magnitudes 6.1 struck the region, USGS reported.
Chile's state copper miner Codelco said workers at its Ventanas division have been evacuated.
