Satellite imagery shows that North Korea is stepping up nuclear activity in line with the regime's vows to expand its weapons program, a US think tank said Thursday.

The Institute for Science and International Security, a private Washington-based research group opposed to nuclear proliferation, said that a December 3 image of North Korea's main Yongbyon nuclear site showed steam at its fuel fabrication complex.

The steam could indicate that the building has gone into operation to produce additional fuel for the aging plutonium reactor, although it can also just come from heating during Korea's bitterly cold winter.

However, the think tank said it saw clear signs of further progress at Yongbyon including construction of what looks like a pool near the centrifuge plant, although the exact purpose of the new structure is unclear.

"Recent commercial satellite imagery shows that North Korea's Yongbyon nuclear center appears to be increasingly active," the think tank said in a report.

The findings are in line with a stream of indications that North Korea has been stepping up its nuclear program. In October, South Korea's spy agency confirmed that the communist state had restarted its five-megawatt plutonium reactor.

The UN nuclear watchdog, the International Atomic Energy Agency, also reported steam on November 28. The agency relies on satellite imagery as North Korea kicked out inspectors in 2009.

North Korea has carried out three nuclear weapons tests, most recently in February, and has vowed to boost what it describes as a deterrent to hostility from the United States.

North Korea has recently also appealed for a resumption of talks, but President Barack Obama's administration has refused talks without indications that Kim Jong-Un's regime is ready to scale down its nuclear program.