VIENNA (Reuters) – North Korea appeared to resume activities this year aimed at producing plutonium, which can be used in the core of an atomic bomb, the U.N. nuclear watchdog has confirmed, though it added that signs of those activities stopped last month.

Pyongyang vowed in 2013 to restart all nuclear facilities, including the main reactor at its Yongbyon site that had been shut down and has been at the heart of its weapons program.

It said last year that Yongbyon was operating and that it was working to improve the “quality and quantity” of its nuclear weapons. It has since carried out what is widely believed to have been its fourth nuclear test.

“From the first quarter of 2016, there were multiple indications consistent with the radiochemical laboratory’s operation,” International Atomic Energy Agency (IAEA) chief Yukiya Amano said in a report to the agency’s annual General Conference, referring to a site used to reprocess plutonium.

“Such indications ceased in early July 2016,” Amano said in the report posted online and dated Friday. Those indications included deliveries of chemical tanks and the operation of a steam plant linked to the lab, the report said.

The IAEA, which has no access to North Korea and mainly monitors its activities by satellite, said last year it had seen signs of a resumption of activity at Yongbyon, including at the main reactor.

There were signs the reactor had been running in the past year, with a pause between October and December, probably to refill it with enough fuel for the next two years, according to the report dated Friday.

Amano said in June that the agency had seen signs of reprocessing, the production of plutonium from spent reactor fuel, at Yongbyon.

Japan’s Kyodo news agency last week quoted North Korea as saying it had resumed plutonium production by reprocessing and had no plans to stop nuclear tests as long as perceived U.S. threats remain.

North Korea’s Atomic Energy Institute, which has jurisdiction over Yongbyon, also told Kyodo it had been producing highly enriched uranium necessary for nuclear arms and power “as scheduled”.

“There were indications consistent with the use of the reported centrifuge enrichment facility,” Amano’s report to the General Conference, which will be held at the end of September, said, adding that construction work had been carried out around the building that houses the facility.

“There were new construction and refurbishment activities on the (Yongbyon) site, which are broadly consistent with (North Korea’s) statement that all the nuclear facilities in Yongbyon have been ‘rearranged, changed or readjusted’,” it added.

(Reporting by Francois Murphy; Editing by Dominic Evans)