Recent satellite imagery suggests that North Korea has restarted a small nuclear reactor, allowing the secretive nation to potentially bolster its stockpile of plutonium for weapons, a U.S. research institute said Thursday.

The North had said five months ago that it would restart key operations at its Yongbyon nuclear facility “without delay.” The report from the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies indicates that it is quietly going ahead with that pledge — and facing few apparent problems in firing up a reactor mothballed for six years.

Commercial satellite images from Aug. 31 show two plumes of white steam rising from a turbine building adjacent to the reactor. That steam is an essential byproduct of the reactor’s operation, and its venting suggests the “electrical generating system is about to come online,” the report said.

The North’s graphite-moderated reactor is ill suited for power production, nuclear experts who have visited the site say, because it has meager capacity and is poorly connected to the electricity grid. But the reactor can be used to produce weapons-grade plutonium — about 13 pounds per year, enough for one or two bombs.

The North said in April that it sought a restart both to offset its “acute shortage of electricity” and to boost its “nuclear armed force both in quality and quantity.”

North Korea rarely allows foreign inspectors access to the Yongbyon site, and there was no immediate way to confirm the reactor’s operation.

The apparent restart, which comes in a period of rapprochement with the South, could raise international pressure to negotiate another freeze of Pyongyang’s nuclear program. But U.S. officials say they are wary of further disarmament talks, noting a pattern in which the North makes promises in return for aid and fuel, then breaks those promises.

United States administrations have struggled over two decades to end North Korea’s nuclear program, using a combination of sanctions and diplomacy. The U.S. has tried to crack down on North Korean financial institutions and companies that serve the country’s weapons program. But the U.S. has also given the North more than $1 billion in aid.

That behavior “makes it very hard to imagine how the six-party [talks] could be fruitful at the moment,” Glyn Davies, the State Department’s envoy for North Korea policy, said in Seoul on Monday. Davies was referring to the multi-nation process for negotiating the North’s denuclearization.

The North has been trying for more than three decades to accumulate fissile material, but the five-megawatt reactor at Yongbyon is the most visible symbol of that program. First started up in 1986, it was frozen twice in diplomatic agreements, most recently in 2007. Experts estimate that North Korea has enough plutonium for four to eight bombs roughly the size of those the United States dropped on Hiroshima and Nagasaki, Japan, in World War II.

The North also has a uranium-enrichment program about which much less is known, as most production facilities are covered or underground — away from satellite detection.

In 2010, the top nuclear engineer at Yongbyon told Siegfried Hecker, a former director of the Los Alamos National Laboratory, that the five-megawatt reactor was in “standby mode” and capable of restarting with just a few months of maintenance work.

Once that reactor restarts, it begins producing plutonium almost immediately.