Activity has been detected at a North Korean nuclear testing site, but South Korean officials say there is no indication of an imminent bomb test, according to various media reports.

Such activity, if true, would come at a very sensitive time. Pyongyang recently declared its readiness to resume nuclear disarmament talks, and leader Kim Jong-il introduced his likely successor.

A US reconnaissance satellite has detected "brisk movements" of vehicles and people, as well as activity to repair a tunnel in the province of North Hamgyong, where it had conducted two earlier tests in October 2006 and May 2009, the Chosun Ilbo newspaper reported, citing an unnamed South Korean government source.

The source ruled out the possibility of an imminent test by the North, as the preparatory work will "take about three months," the report said. CBS News also quoted government sources as saying the threat wasn't imminent.

An unnamed South Korean defense ministry official was quoted by AFP as saying that such movements were being constantly detected and were possibly for the daily maintenance of key strategic facilities at the site.

A spokesman for South Korea's Joint Chiefs of Staff, however, downplayed the report, saying there was no evidence of any such preparations, but added, "We're watching closely any development concerning the North's nuclear facilities and sharing information with countries concerned."

The North might give the impression that it is constantly ready to perform another nuclear test to pressure South Korea and the US to ease sanctions, the report added.

In defiance of international pressure, North Korea conducted its first nuclear test in October 2006 and followed it up with a second one in May last year in Punggye-ri in North Hamgyong province, a month after it walked out of the Six-Party Talks, which involved the two Koreas, the US, China, Japan and Russia, following a UN condemnation of its missile tests.

The Foreign Ministry of China and the North Korea embassy in Beijing were unavailable Thursday for immediate comment on the Chosun Ilbo report.

The report came shortly after Kim Jong-un, the youngest son of Kim Jong-il, was introduced as the likely successor to the leadership. The junior Kim was made a four-star general and promoted to a key post last month.

There is speculation that the North may bolster nuclear weapons in line with the country's military-first policy to consolidate the younger Kim's succession, South Korea media reports said.

"Taking Stock: North Korea's Uranium Enrichment Program," a report released ealier by the Institute for Science and International Security, a US think tank, said that North Korea "has moved beyond laboratory-scale work" and has the capability to build a "pilot-scale" plant to enrich uranium.

Over the weekend, North Korea, however, expressed its willingness to resume the six-nation disarmament talks, but would not be "hasty" because the US and some other parties were "not ready."