A North Korean official has apparently told Chinese authorities that the communist state is planning to conduct a third nuclear test in the coming week, a news report said Saturday.

"We've heard a North Korean official in Beijing told the Chinese side that the North planned to carry out a nuclear test between January 13-20," the Joongang Ilbo daily quoted an unidentified Seoul official as saying.

South Korean officials have a policy of not commenting on intelligence matters.

"We're now stepping up surveillance over the Punggye-ri nuclear test site," the official said in reference to the North's only nuclear test site, where tests were carried out in 2006 and 2009.

With the UN Security Council still debating possible sanctions against the North following the launch of a long-range rocket last month, there has been widespread speculation that Pyongyang may carry out a third nuclear test.

However, Professor Yang Moo-Jin of the University of North Korean Studies in Seoul said there were "no signs of a nuclear test being imminent".

"Chances are slim that the North might push ahead with a nuclear test in this winter season, especially when China is insisting on a moderated response to the rocket launch to prevent a third nuclear test taking place," Yang told AFP.

Last month a US think-tank citing satellite photos said the North had repaired extensive rain damage at the nuclear test site in the northeast of the country and could conduct a detonation on two weeks' notice.

The US-Korea Institute at Johns Hopkins University said satellite photos as recent as December 13 showed Pyongyang was determined to maintain a state of readiness at Punggye-ri.

South Korea's Unification Minister Yu Woo-Ik told a parliamentary committee last month it was "highly probable" the North would likely follow up the successful rocket launch with another nuclear test.

"Judging from analysis of intelligence, significant preparations have been made," he said.

North Korea had a track record of conducting nuclear tests following missile launches, which were aimed at developing a delivery system for nuclear warheads, Yu said.

The North's previous nuclear tests were both carried out within months of long-range rocket launches.

Pyongyang insists the launch was a purely scientific mission aimed at placing a polar-orbiting earth observation satellite in space, but most of the world saw it as a disguised ballistic missile test.