Nuclear radiation monitoring in South Korea. North Korea is suspected of carrying out a nuclear test after an earthquake was detected. Photograph: Yonhap/EPA
There are signs that North Korea has conducted a threatened nuclear test, with an earthquake of magnitude 4.9 detected near the underground site where it was expected to carry out the explosion.
The US Geological Survey said on Tuesday it had detected the earthquake in North Korea but neither Pyongyang nor Seoul confirmed whether the widely anticipated third nuclear test had happened. South Korean reports described the quake as "manmade".
The South Korean defence ministry said it was trying to determine whether North Korea had conducted a nuclear test. Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.
The quake occurred at 11.57am Korean time (2.57am GMT) and South Korea's presidential office said that it was "likely" a nuclear test, according to the South's Yonhap news agency.
North Korea is not prone to seismic activity.
The USGS said the epicentre of the quake was a kilometre underground and close to the North's known nuclear test site.
In Vienna, international monitoring group the Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) said it was analysing an "unusual seismic event" in North Korea.
The CTBTO is an independent body that monitors for nuclear tests and has 270 facilities around the world that check for quakes, radioactive particles in the atmosphere and other evidence.
North Korea's politburo vowed to continue firing "powerful long-range rockets" in a statement on Tuesday that made no mention of Pyongyang's promise to conduct a nuclear test.
The United States and its allies have been on edge since North Korea said last month it would conduct its third nuclear test in protest at sanctions that were toughened when it launched a rocket in December. The UN called the launch a cover for a banned missile test.
North Korea's powerful National Defence Commission said on 23 January that the United States was its prime target for a nuclear test and long-range rocket launches. North Korea accuses Washington of leading the push to punish Pyongyang for its December rocket launch.
In October 2012 a spokesman from the commission told state media that the country had built a missile capable of striking the United States but he did not provide further details. A missile featured in an April 2012 military parade appeared to be an intercontinental ballistic missile but its authenticity has not been verified by foreign experts.