North Korea may have temporarily shut down a plutonium reactor earlier this year as it grappled with water supply problems that could threaten the safety of its nuclear complex, a U.S. research institute said Monday.

The North restarted the 5-megawatt reactor at the Nyongbyon complex only last summer, backtracking on a commitment of previous aid-for-disarmament negotiations and raising fresh alarm over its nuclear weapons program.

A constant water supply from a nearby river is essential to cool the reactor, which can produce fissile material for bombs, and would be needed for the safe operation of another reactor under construction at the same complex.

U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says recent commercial satellite imagery show the recently restarted reactor may have been temporarily shut down or operated at a lower power level for repairs after flooding caused the river to change course.

The reactor appears to have been operating again by mid-February, according to the analysis by Nick Hansen, a retired intelligence expert who closely monitors developments in the North's weapons programs. The findings were published on the institute's website, 38 North.

The safety of the North's nuclear facilities adds to international worries over Pyongyang's nuclear ambitions.

North Korea has conducted three nuclear test explosions, the latest in February 2013, and recently warned, without elaborating, it may carry out a "new form of nuclear test." It also recently test-launched two medium-range ballistic missiles and exchanged artillery fire at its sea border with South Korea.

International negotiations on denuclearization have been stalled since 2009, and envoys of the U.S., Japan and South Korea were meeting in Washington on Monday to discuss policy toward the North.

In late March, South Korean President Park Geun-hye said that because of the concentration of nuclear facilities at Nyongbyon, a fire there could cause a disaster potentially worse than the nuclear meltdown at Chernobyl in 1986.

Analysts for 38 North dispute that claim because of the relatively small sizes of the reactors at Nyongbyon, but warn that a failure of the cooling system could cause a fire in the graphite core of the 5-megawatt reactor and release radioactivity into the atmosphere and the nearby Kuryong River.

They say the risks are potentially greater for a 30-megawatt experimental lightwater reactor once it begins in operations in perhaps one or two years because the North lacks experience operating such a reactor. Sufficient water supply is another major concern. Judging from overhead imagery, the two reactors depend on the same cooling system using water pumped from the river.

Flooding in July 2013 apparently led to the silting up of cisterns at the riverside used to collect water for cooling, so North Korea installed temporary pipes to draw water from a different location in the river while a new pipe to a cistern was constructed. That provided a fix but doubts remain over whether the river can provide a reliable, year-round water supply.

"These recent problems should be a wake-up call for Northeast Asia," said Joel Wit, a former State Department official and editor of 38 North.

"Whether the North Koreans will be able to meet the challenge of providing a reliable, adequate source of water to cool the Nyongbyon reactors, particularly when the experimental light-water reactor becomes operational, could pose a serious problem for both Pyongyang and its neighbors," he said.