Activities observed at North Korea's Yongbyon site indicate testing ahead of a possible restart to a reactor that could provide it with weapons-grade plutonium, the UN atomic agency said Thursday.

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

He told the board the IAEA "continues to monitor developments at the Yongbyon site, mainly through satellite imagery".

At a later news conference Amano said the reactor activities included the release of steam from two vents and the apparent discharge of cooling water.

"As we do not have inspectors on the ground we cannot draw a conclusion," Amano said.

North Korea kicked out IAEA inspectors in 2009.

Since 2006 it has carried out three nuclear tests, the last, and most powerful, in February this year.

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

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.