North Korea's "belligerent behavior" is destabilizing an already volatile region of east Asia, the US military's top officer said Sunday after a report that Pyongyang has built a new nuclear plant.

"From my perspective, it's North Korea continuing on a path which is destabilizing for the region," Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, told CNN's "State of the Union" show.

"It confirms or validates the concern we've had for years about their enriching uranium which they've denied routinely and that when I look at this, it is consistent with belligerent behavior, and the kind of instability creation in a part of the world that is very dangerous," Mullen added.

North Korea has tested two nuclear devices in the past four years, triggering grave international concern that the impoverished state made good on its claim that it has the bomb.

The United States had been engaged in six-party talks with other world powers including key Pyongyang ally China, but North Korea quit the talks last year, just a month before its second nuclear test.

The regime also announced last year it was restarting its Yongbyon nuclear complex, outside the North Korean capital, after UN condemnation and sanctions.

In the startling report in Sunday's New York Times, Stanford University scientist Siegfried Hecker revealed that he toured the new facility last week at Yongbyon and said he was astonished by what he found.

"Instead of seeing a few small cascades of centrifuges, which I believed to exist in North Korea, we saw a modern, clean centrifuge plant of more than a thousand centrifuges all neatly aligned and plumbed below us," the Times quoted him as saying.

US senior lawmaker John Kerry, chairman of the Senate Foreign Relations Committee, described the report as "troubling" and said it underscored the need to resume the six-party talks on denuclearization of the Korean Peninsula.

"North Korea's activities should also prompt the UN to reexamine the effectiveness of efforts to implement existing sanctions against the North's nuclear program," Kerry said.

"Only a comprehensive approach that can achieve security, peace, and development offers any hope of verifiably eliminating North Korea's nuclear weapons," Kerry added.

"The longer it takes to launch that effort, the longer the United States and its allies will be forced to cope with the destabilizing consequences of the DPRK's (North Korea's) reckless and irresponsible pursuit and export of highly-sensitive technologies."

North Korea said in September last year that it had reached the final stage of enriching uranium -- a second way of making nuclear bombs on top of its plutonium-based program.