The agency published satellite images taken by Global Security (www.globalsecurity.org) on January 5 of the Yongbyon site.

Compared with photos taken previously, the latest images showed not only new vehicles, containers and paved roads but also thicker plumes of smoke from the reactor chimney.

"The satellite photos ... strongly indicate that the reactor is in full operation," Yonhap said.

The Yongbyon site is at the center of a standoff over the Stalinist state's nuclear ambitions because its five-megawatt reactor is suspected of producing plutonium for nuclear weapons.

US and South Korean intelligence believes the reactor is capable of producing at least six kilograms (13.2 pounds) of plutonium -- needed to make one nuclear bomb -- every year, according to Yonhap.

The Yongbyon reactor was mothballed after a 1994 US-North Korean agreement on curbing the nuclear programme. It was reactivated in February 2003 following a renewed nuclear crisis.

North Korea said in February 2005 that it had built nuclear weapons.

Six-nation talks aimed at ending its nuclear weapons drive have been stalled since November due to the North's boycott of the negotiations in protest at US financial sanctions.

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