North Korea appears to have restarted a reactor that produces plutonium, making good on threats to boost its stockpile of nuclear weapons, US analysts said.

Satellite imagery taken on August 31 shows white steam coming out of a building next to the five-megawatt plutonium reactor at Yongbyon, the US-Korea Institute at Johns Hopkins University said on Wednesday.

The image shows that North Korea "appears to have put the reactor into operation," researchers Nick Hansen and Jeffrey Lewis wrote on the institute's blog, 38 North.

The reactor "is capable of producing six kilograms of plutonium a year that can be used by Pyongyang to slowly increase the size of its nuclear weapons stockpile," it said.

North Korea had declared in April that it would restart all facilities at Yongbyon to "bolster the nuclear armed force both in quality and quantity."

The pledge came at a period of high international tension over North Korea, which defiantly carried out a third nuclear test in February and threatened to attack the United States over its reaction.

Tensions have since eased somewhat, but the United States has been skeptical of resuming talks with North Korea without proof that Kim Jong-Un's regime is committed to ending its nuclear program.

North Korea had shut down its Yongbyon reactor in July 2007 under a six-nation aid-for-disarmament deal and publicly knocked down its cooling tower to demonstrate its commitment.

The reactor was the totalitarian state's sole way of producing plutonium, which it used to conduct its first two nuclear tests in 2006 and 2009.

Foreign governments and experts have not been able to conclude whether North Korea again used plutonium in its February test, but the regime is known to be working to produce uranium to offer a second way to a bomb.

The Institute for Science and International Security, another US think tank that has closely followed North Korea, reached similar conclusions after observing the steam from Yongbyon.

The institute said that the restart of Yongbyon did not preclude a new diplomatic effort, saying that North Korea would still need two to three years to discharge irradiated fuel containing plutonium.

"There remains time to negotiate a shutdown of the reactor before North Korea can use any of this new plutonium in nuclear weapons," David Albright and Robert Avagyan wrote in an analysis for the institute.

"If a shutdown is achieved in the next six months, the reactor would have produced very little plutonium," they said.

The revelations came as Glyn Davies, the US pointman on North Korea, toured the region in the latest diplomacy by Washington to plot the next move.

"Right now, we simply do not see the positive attitude of North Korea toward fulfilling its obligations, its commitments, to living up to UN Security Council Resolutions, and we need to see that," Davies said Tuesday in Seoul.

"We remain open, of course, to dialogue with North Korea. As a diplomat, I would like very much to get back to that, but I think it is important that we only do so when the conditions are right," he said.

His trip came days after a regional tour by Robert King, the US envoy for human rights in North Korea.

King had planned to go to Pyongyang to seek the release of Kenneth Bae, a detained US citizen whose health is apparently deteriorating. But US officials said that North Korea rescinded King's invitation at the last minute.