North Korea is preparing to carry out a fourth nuclear test "at any moment", South Korea's Ministry of Defence has said.
Seoul's warning came a few days ahead of US President Barack Obama's planned visit to South Korea.
"Our military is currently detecting a lot of activity in and around the Punggye-ri nuclear test site," South Korea Defence Ministry spokesman Kim Min-seok told a press briefing.
Kim declined to give details of the monitored activity, but warned that it might be a "deception tactic" to raise tensions ahead of Obama's trip.
"We are thinking of possibilities that the North may stage a surprise nuclear test or just pretend to stage a nuclear test," Kim added.
The US president's visit has been strongly criticised by Pyongyang, which slammed it as a dangerous move that would escalate military tension between the two Koreas.
The South Korean spokesman added that Seoul and US militaries were closely sharing intelligence and Seoul's Joint Chiefs of Staff had set up a special task force in case Pyongyang carries out the drill.
However, despite concerns, satellite images have shown that a North Korea nuclear test is unlikely.
US-Korea institute 38 North, which provides analysis of events in and around North Korea, confirmed that there had been an increase in activity, but added that were few signs of an imminent test.
"Recent operations at Punggye-ri have not reached the high level of intensity – in terms of vehicle, personnel and equipment movement – that occurred in the weeks prior to past detonations," it said.
"Moreover, other possible indicators present before the North Korean nuclear tests in 2009 and last year, such as communications vans and a satellite dish intended to transmit pre-test data, have not been spotted."
Professor Yang Moo-jin of the University of North Korean Studies in Seoul advised that a test now would risk permanently alienating the North's only major ally, China.
"It would be a huge slap in the face for China and North Korea may not feel confident enough to deal with the backlash from Beijing."
Kim Yong-hyun, a North Korean expert at Dongguk University, said: "The diplomatic backlash from another nuclear test might be hard for the North to cope with.
"I think this is more likely North Korea posturing to get some international attention."
North Korea has already conducted three nuclear tests: in 2006, 2009 and 2013.
The country has been criticised for firing medium-range missiles over the sea just as Japan, South Korea and the US were sitting down to talks aimed at resolving tensions in the region.