WASHINGTON (AFP) - North Korea appears to have restarted a reactor that produces plutonium, making good on threats to boost its stockpile of nuclear weapons, a US think tank said Wednesday.

Satellite imagery taken on Aug 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.

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.