North Korea appears to have finished extracting plutonium from spent nuclear fuel, gaining up to four more nuclear weapons worth of plutonium, a US research institute said Tuesday.

The Institute for Science and International Studies (ISIS) said that satellite imagery taken of the North's Yongbyon nuclear complex between July and August shows little activity at the North's nuclear reprocessing plant, compared with brisk activity earlier this year when reprocessing was believed to be under way.

"The recent campaign to separate plutonium at the Radiochemical Plant may be finished," ISIS said in a report. "An earlier Institute report estimated that North Korea could have separated about 5.5 to 8 kilograms of plutonium during this campaign, which is roughly enough for 1 to 4 nuclear weapons."

The institute said that imagery also shows continued activity at the complex, such as vehicle movements, but the 5-megawatt nuclear reactor at Yongbyon, the main source of plutonium, appears to be operating sporadically.

The "imagery dated August 26, 2016 showed no outward signs of full power, such as extensive water discharge into the nearby river or steam venting from the reactor's turbine structure," it said.

"Our assessment is that the reactor has operated intermittently or at low power since mid-2014. Based on the recent imagery, we do not detect evidence of consistent operation at full reactor power in July and August 2016," the institute said.

"Although North Korea has conducted extensive renovations of the reactor during the last several years, including importing equipment and materials from abroad, the reactor's operation may remain sporadic," it said.

The institute said that construction of a new light water reactor at Yongbyon appears to be still on going. But once completed, the new reactor will be a bigger source of weapons-grade plutonium for the North than the 5-megawatt reactor.

"Given the importance of plutonium to North Korea's nuclear weapons program, another question remains whether North Korea will seek to boost plutonium production at these two reactors by getting the smaller one to work better and starting the larger one," it said. (Yonhap)