North Korea has been carrying out construction work at its Yongbyon nuclear complex, including near a mothballed reactor that experts say could produce plutonium for bombs, a U.N. nuclear agency report showed on Wednesday.
The U.N. watchdog, which monitors the isolated state's nuclear developments via satellite, said the activities appeared to be broadly consistent with the North's "statements that it is further developing its nuclear capabilities."
North Korea's nuclear program "remains a matter of serious concern," the International Atomic Energy Agency (IAEA) report to member states said.
Pyongyang announced in April that it would revive the aged Yongbyon five-megawatt research reactor that yields bomb-grade plutonium, but stressed it was seeking a deterrent capacity.
Nuclear experts said at the time it would probably take about half a year to get the reactor up and running if it had not suffered significant damage from neglect.
The Yongbyon reactor has been technically out of operation for years. In 2008 the North destroyed its cooling tower as a confidence-building step in U.S.-led multilateral negotiations aimed at reducing tensions on the Korean peninsula.
But the thaw in tensions was short-lived. Six-nation aid-for-disarmament talks between the two Koreas, China, Russia, Japan and the United States have been stalled for years.
North Korea said in July it would not give up its nuclear deterrent until Washington ends its "hostile policy" towards Pyongyang, although it was ready to revive nuclear talks. The United States fought on the side of the South in the 1950-53 Korean War, which ended in a truce, not a peace treaty.
North Korea — which withdrew from the nuclear Non-Proliferation Treaty in 2003 — conducted its third nuclear test in February this year, prompting stiffer U.N. sanctions.
The IAEA report suggested North Korea may have enough uranium for a "full core load" of fuel but it was "not possible to determine when the reactor may start operation."
It said construction activities on buildings adjacent to the reactor building and the excavation of trenches in the vicinity were observed between March and June.
"These trenches appear to be related to the reconfiguration of the reactor's cooling system," it said. "If this is the case, such a reconfiguration could possibly enable the reactor to be restarted without rebuilding the cooling tower."