North Korea could be preparing to carry out a fourth nuclear test, South Korea said today.
It cited increased activity at its main test site, just days ahead of a visit to Seoul by US President Barack Obama.
"Our military is currently detecting a lot of activity in and around the Punggye-ri nuclear test site," Defence Ministry spokesman Kim Min-Seok told a press briefing.
Mr Kim stressed that North Korea's nuclear weapons programme was at a stage where it could conduct a test "at any moment" once the order was given by the leadership in Pyongyang.
North Korea has conducted three nuclear tests, in 2006, 2009 and 2013, all at the Punggye-ri site in the northeast of the country.
Mr Kim declined to give details of the monitored activity.
However, he cautioned that it may be no more than a "deception tactic" to raise tension ahead of Mr Obama's visit, which is due to begin on Friday.
"We are thinking of possibilities that the North may stage a surprise nuclear test or just pretend to stage a nuclear test," Mr Kim said.
Mr Obama is visiting Seoul as part of an Asia tour and there has been widespread speculation that the North may stage a provocation to coincide with the trip.
Mr Kim said the South Korean and US militaries were closely sharing intelligence and Seoul's Joint Chiefs of Staff had set up a special task force in case North Korea goes ahead with an underground detonation.
"We plan to thoroughly prepare for a fourth nuclear test or any other form of provocation," Mr Kim said.
Yesterday, North Korea slammed Mr Obama's upcoming trip as a "dangerous" move that would escalate military tension and bring the "dark clouds of a nuclear arms race" over the Korean peninsula.
Several analysts said they were sceptical that North Korea would carry out a test at the current time, and said it was just seeking to rattle a few cages during the Obama visit.
The North warned late last month that it would not rule out a "new form" of nuclear test after the UN Security Council condemned its latest series of medium-range missile launches.
US 'closely watching' for signs of test
Elsewhere, the US said it was watching the situation "very closely" following the South Korean warnings.
"North Korea has a history of taking provocative action and we are always mindful of the possibility that such an action could be taken," said White House spokesman Jay Carney.
Mr Carney said any action by North Korea would "most likely be in violation of numerous commitments that the DPRK (North Korea) is bound by, but of course that is something that they unfortunately have done many times".
The South Korean claims were backed up to some degree by the US-Korea Institute at Johns Hopkins School of Advanced International Studies.
It said that recent commercial satellite images showed "new operations" at Punggye-ri.
Imagery from March to as recently as Saturday show the movement of crates, boxes and possibly wood near the entrances, according to analysis posted on the institute's website, 38 North.
"It is possible these materials are being moved inside those tunnels," the institute said.
But it qualified the discoveries by saying the activity "falls short" of what was observed there in the weeks prior to past detonations.
It "could represent an early stage of preparations for a test or may be intended for a less provocative purpose, such as conducting maintenance after a long winter".