VIENNA (AFP) -- Activities observed at North Korea's Yongbyon site indicate an effort to restart a reactor that could provide it with weapons-grade plutonium, the UN atomic agency said Thursday.

"The Agency continues to monitor developments at the Yongbyon site, mainly through satellite imagery," International Atomic Energy Agency head Yukiya Amano said.

"Activities have been observed at the site that are consistent with an effort to restart the ... reactor," Amano told a closed-door regular meeting of the IAEA's board of governors.

"However, as the Agency has no access to the site, it is not possible for us to conclusively determine whether the reactor has been restarted," he said, according to the text of his speech provided by the IAEA.

North Korea kicked out IAEA inspectors in 2009.

South Korea's spy agency the National Intelligence Service (NIS) said on October 8 in a report to parliament that the reactor had resumed operations.

This followed the release of satellite imagery by the US-Korea Institute at Johns Hopkins University and the Institute for Science and International Security showing activity at the site.

Yongbyon was shut down in July 2007 and Pyongyang began disabling key plants there, publicly demolishing the plutonium reactor's cooling tower in 2008.

Pyongyang carried out its third nuclear test in February, sparking international condemnation and raising tensions on the Korean peninsula for months.

Two months later, it boasted that it would restart all facilities at Yongbyon to bolster its atomic arsenal.