VIENNA, Dec 22 (Reuters) - A reactor at North Korea's Yongbyon nuclear complex appears for the first time to be using atomic fuel, the U.N. nuclear watchdog said late on Thursday, which would mean the country has a new potential source of plutonium for its nuclear weapons.

North Korea has for years used spent fuel from a 5 megawatt nuclear reactor at Yongbyon to produce plutonium for its nuclear arsenal but a telltale discharge of warm water from a larger light-water reactor suggests that is now coming online, too, the International Atomic Energy Agency said.

"The discharge of warm water is indicative the reactor has reached criticality," IAEA chief Rafael Grossi said in a statement, meaning the nuclear chain reaction in the reactor is self-sustaining.

The IAEA has not had access to North Korea since Pyongyang expelled its inspectors in 2009. The agency now observes the country from afar, mainly using satellite imagery. Without access, the IAEA could not confirm the reactor's operational status, Grossi said.

The IAEA says it has observed a strong outflow of water from the light-water reactor's cooling system since October, suggesting ongoing commissioning of the reactor. More recent indications were that the water was warm, Grossi said.

"The LWR, like any nuclear reactor, can produce plutonium in its irradiated fuel, which can be separated during reprocessing, so this is a cause for concern," he said, adding that the advancement of North Korea's nuclear programme was "deeply regrettable".

The light-water reactor had appeared complete for years without it going into operation.

Activity at North Korea's nuclear test site at Punggye-ri has led to months of speculation that it is preparing to test another nuclear weapon as it seeks to miniaturise its warheads for use in its ballistic missiles. North Korea has conducted six nuclear tests.

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