An earthquake has been detected in North Korea, close to a site previously used for nuclear tests.
China, Japan and South Korea have all said there are indications the tremor was man-made, meaning the North may have carried out a new nuclear test.
North Korean state media has said a "special, significant" announcement will be made shortly.
South Korean ministers are holding an emergency meeting, the Yonhap state news agency said.
North Korea is believed to have conducted three underground nuclear tests since 2006, all at a site called Punggye-ri.
The new tremor was detected about 50km (30 miles) from Punggye-ri at a depth of about 10km, according to the US Geological Survey.
The USGS said the quake had a magnitude of 5.1. South Korea's meteorological agency measured it at 4.2.
Japan's chief cabinet secretary Yoshihide Suga said that "considering past cases, there is the possibility that this might be a nuclear test by North Korea".
North Korea's nuclear tests
9 October 2006
The first test exploded a device based on plutonium, rather than enriched uranium. The test was conducted at P'unggye-yok. US intelligence officials said that it had not been a powerful one, measuring less than one kiloton - that is less then a tenth of the size of the bomb dropped on Hiroshima.
25 May 2009
A second underground nuclear test is carried out which was said to be more powerful than the first. Russia's defence ministry estimated a blast of up to 20 kilotons, a similar size to the American bombs that completely destroyed Hiroshima and Nagasaki in 1945.
Although the North gave no details of the test location, South Korean officials said a seismic tremor was detected in its neighbour's north-east around the town of Kilju - close to P'unggye-yok.
12 February 2013
Unusual seismic activity was detected around the Punggye-ri underground nuclear test site. This was followed by confirmation from the state news agency that North Korea had successfully tested a device. The announcement referenced a "miniaturised" nuclear device which raised fears that Pyongyang's ultimate aim is to produce a device small enough to fit on a long-range missile.