North Korea could be preparing to carry out a fourth nuclear test, South Korea says, ahead of a visit by US president Barack Obama to Seoul.

South Korea's defence ministry says there has been a significant increase in activity at North Korea's Punggye-ri nuclear test site.

Defence spokesman Kim Min-Seok says a special taskforce has been set up in case Pyongyang 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.

However, Mr Kim says they are not ruling out the possibility that the North may "fake a test" to coincide with Mr Obama's visit to South Korea on Friday.

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.

On Monday, Pyongyang slammed Mr Obama's upcoming trip as a "dangerous" move that would escalate military tension on the Korean peninsula.

North Korea has conducted three nuclear tests - in 2006, 2009 and 2013 - at the Punggye-ri site in the northeast of the country.

'Posturing' from Pyongyang

Analysts say they are sceptical that North Korea would carry out a test at the current time.

Professor Yang Moo-Jin, of the University of North Korean Studies in Seoul, says a test now would risk permanently alienating the North's only major ally and chief economic benefactor, 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," Mr Yang said.

A nuclear test would extinguish any chance of a resumption of six-country talks on North Korea's nuclear program that China has been pushing for.

Other parties to the stalled discussions - most notably a sceptical South Korea and the US - insist Pyongyang must first make a tangible step towards denuclearisation.

"The diplomatic backlash from another nuclear test might be hard for the North to cope with," said Kim Yong-Hyun, a North Korean expert at DongGuk University.

"I think this is more likely North Korea posturing to get some international attention," Mr Kim said.

Last month Pyongyang warned 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.

Experts saw this as a possible reference to testing a uranium-based device or a miniaturised warhead small enough to fit on a ballistic missile.

AFP