North Korea appears to have restarted plutonium production for its nuclear weapons program at its main Nyongbyon reactor complex, commercial satellite imagery released by a think tank showed Friday.

Imagery from a previous analysis by the North Korean-watching website 38 North on Jan. 18 showed that Pyongyang had been preparing to restart the reactor after spent fuel rods had been unloaded for reprocessing to produce extra plutonium for its weapons program, the report said.

“Imagery from January 22 shows a water plume (most probably warm) originating from the cooling water outlet of the reactor, an indication that the reactor is very likely operating,” the report said, noting that most of the river remained frozen except where the warm water mixed.

“Currents carry this mix downstream — visible as a plume of ice-free water.”

However, without being able to measure the water’s temperature rise or flow from the reactor, the report said it was impossible to estimate the power level the reactor was running at — “although it may be considerable.”

According to the earlier report by 38 North, operations at the Nyongbyon complex had been halted since late 2015.

Pyongyang has stoked international concern with its atomic saber-rattling. In a his New Year’s address, North Korean leader Kim Jong Un said that the country had “entered the final stage of preparation” for the test-launch of an intercontinental ballistic missile.

Experts say that the isolated nation is moving closer to making this claim a reality after conducting an unprecedented spate of missile and nuclear tests over the past year — including its most powerful atomic blast to date.

Earlier this month, South Korea’s Yonhap news agency reported that two new missiles had been loaded onto mobile launchers, citing high-level South Korean officials and South Korean and U.S. military sources.

The missiles were thought to be equipped with new engines that Pyongyang tested last April, Yonhap reported. The North said at the time that the new tech would “guarantee” an eventual nuclear strike on the U.S. mainland.

According to Yonhap, the missiles’ existence appeared to have been intentionally leaked by Pyongyang to send a “strategic message” to the administration of new U.S. President Donald Trump.

Trump has said that a launch by the North of a nuclear weapon capable of hitting U.S. shores “won’t happen,” though he has not given specifics as to how he would halt Pyongyang’s progress toward mastering the technology.

On Wednesday, a senior diplomat in Pyongyang reiterated the North’s claims that it is ready to test-fire an ICBM “at any time, at any place.”

Choe Kang Il, deputy director general for North American affairs at the North’s Foreign Ministry, made the threat in an interview with NBC TV, saying such a move would be “defensive in nature.”

The North has been known to conduct weapons tests around key dates, and some experts believe it could test an ICBM to mark the late leader Kim Jong Il’s birthday on Feb. 16.

Friday’s revelation of increased activity at the Nyongbyon plant comes on the heels of an announcement that U.S. Defense Secretary James Mattis will visit Japan and South Korea in the coming week. Discussions are expected to focus on bolstering alliances with the two Asian giants amid the North Korean threat.

“The trip will underscore the commitment of the United States to our enduring alliances with Japan and the Republic of Korea, and further strengthen U.S.-Japan-Republic of Korea security cooperation,” the Pentagon said in a statement.