VIENNA, June 6 (Reuters) - North Korea appears to have reopened a plant to produce plutonium from spent fuel of a reactor central to its atomic weapons drive, the U.N. nuclear watchdog said on Monday, suggesting the country's arms effort is widening.

Pyongyang vowed in 2013 to restart all nuclear facilities, including the main reactor at its Yongbyon site that had been shut down and has been at the heart of its weapons programme.

It said in September that Yongbyon was operating and that it was working to improve the "quality and quantity" of its nuclear weapons. It has since carried out what is widely believed to have been a nuclear test.

The International Atomic Energy Agency (IAEA), which has no access to North Korea and mainly monitors its activities by satellite, said last year it had seen signs of a resumption of activity at Yongbyon, including at the main reactor.

"Resumption of the activities of the 5 megawatt reactor, the expansion of centrifuge-related facility, reprocessing, these are some of the examples of the areas (of activity indicated at Yongbyon)," IAEA chief Yukiya Amano told a news conference during a quarterly IAEA Board of Governors meeting.

Centrifuges are machines that enrich uranium, a process that can purify the element to the level needed for use in the core of a nuclear weapon. Reprocessing involves obtaining plutonium from spent reactor fuel, the other main route to a bomb.

"There are indications the reprocessing plant at Yongbyon has been reactivated," an IAEA spokesman said later on Monday. "It is possible that it is reprocessing spent fuel."

Little is known about the quantities of weapons-grade uranium or plutonium that North Korea possesses, or its ability to produce either, though plutonium from spent fuel at Yongbyon is widely believed to have been used in its nuclear bombs.

North Korea has come under tightening international pressure over its nuclear weapons programme, including tougher U.N. sanctions adopted in March backed by its lone major ally China, following its most recent nuclear test in January.

The website 38 North reported in April that exhaust plumes had been detected on two or three occasions in recent weeks from the thermal plant at Yongbyon's Radiochemical Laboratory, the site's main reprocessing installation.

The U.S. national intelligence director said in February that North Korea could be weeks away from recovering plutonium from Yongbyon, telling the Senate Armed Services Committee that it had also expanded its uranium enrichment facility there.

(Reporting by Francois Murphy; Editing by Tom Heneghan)

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