South Korean President Lee Myung-Bak believes North Korea could detonate multiple devices when it goes ahead with a nuclear test expected in the coming weeks or even days.

In an interview published Tuesday in the Chosun Ilbo daily, the outgoing president also acknowledged the huge challenge the international community faces in seeking to wean Pyongyang off its nuclear weapons programme.

The North has signalled that it will carry out a "higher level" nuclear test very soon, in a defiant response to tightened UN sanctions after its successful long-range rocket launch in December.

Lee said "higher-level" suggested Pyongyang might attempt to detonate several devices.

"North Korea is likely to carry out multiple nuclear tests at two places or more simultaneously" to maximise scientific gains from an event that will be globally condemned, Lee said.

Experts around the world are gearing up to analyse any test for what it might reveal about the current status of the North's weaponisation programme.

Of particular interest will be any sign that its scientists have succeeded in developing a warhead that can be fitted onto a missile.

"If the North produces miniaturised weapons that can be used as warheads on missiles, it would really pose a threat," Lee said. "That's why the whole world is watching it so intensively."

Lee has only a few weeks left in office at the end of a five-year term marked by an almost total freeze of contacts between North and South Korea.

In his interview, he suggested that diplomatic efforts would make little headway in bringing about a significant policy shift in Pyongyang.

"I think it is difficult to persuade the North regime to give up the nuclear path," he said.

Some predict the test will come before the Lunar New Year on February 10, while others suggest it will be timed to coincide with the February 16 birthday of late leader Kim Jong-Il, father of current leader Kim Jong-Un.

South Korea and its ally the United States are holding a joint naval exercise off the Korean peninsula as tensions rise -- a drill condemned as a "warmongering" exercise by North Korea.

Its official news agency Tuesday threatened a move going "beyond the imagination" of Seoul and Washington in response to the exercise, which it terms a rehearsal for invasion.

"The US is now getting hell-bent on the joint anti-submarine drill targeting the 'north's nuclear facilities'," the agency said in an editorial.

"Now that the hostile acts toward the (North) have gone beyond the limit of universally accepted... norms of the international community, the (North's) option in reaction to it will also go beyond the imagination of the hostile forces," it said.

"There is no other option for the (North) but to fight it out."