North Korea has already begun work to restart a plutonium production reactor in a sign that its confrontational rhetoric may not all be bluster, a US think tank said Wednesday.

The US-Korea Institute at Johns Hopkins University said that a satellite photograph seen on March 27 indicated construction at the plutonium reactor at Yongbyon, which was shut down in 2007 as part of a US-backed agreement.

The analysis comes one day after North Korea announced it would restart all facilities at Yongbyon, one of a string of recent bellicose statements that included a warming that it will attack the United States with nuclear weapons.

Writing on the institute's 38 North blog, researchers said that the photograph showed what appeared to be construction along a road and near the back of the reactor building aimed at restoring vital cooling functions.

The construction may indicate that the North Koreans are connecting a secondary cooling system at the reactor to a facility built for a separate light water reactor that is located nearby.

Such a move is necessary because North Korea in 2008 demolished the cooling tower in a bid to give visible proof of its denuclearization as it tried to seal an accord with then US president George W. Bush's administration.

Analysts Nick Hansen and Jeffrey Lewis wrote that the construction would offer a faster way to restore production at the reactor, which can produce 13 pounds (six kilograms) of plutonium a year once operational.

North Korea, led by young leader Kim Jong-Un, has vowed to restore its nuclear weapons program and to attack the United States and its allies as part of a worsening crisis.

North Korea voiced outrage over US-led criticism of a rocket launch it carried out in December that put a small satellite into orbit. The regime undertook its third nuclear test in February.