"The continuation and further development of the DPRK's nuclear programme and related statements by the DPRK are a cause for grave concern," said a report by the International Atomic Energy Agency (IAEA), referring to North Korea's official name.

The report, published late Monday, by director general Yukiya Amano is to be submitted to an IAEA board meeting in September.

In 2009 Pyongyang expelled IAEA inspectors from its Yongbyon nuclear site and has since refused to allow IAEA inspections on its territory.

The watchdog has stepped up monitoring through open source information and satellite imagery, it said.

"As the Agency remains unable to carry out verification activities in the DPRK, its knowledge of the DPRK's nuclear programme is limited and, as further nuclear activities take place in the country, this knowledge is declining," it said.

Between late-April and early-May, there were indications of the operation of the steam plant that serves the radiochemical laboratory at the Yongbyon site, according to the report.

However, the steam plant was not operative long enough to have supported the reprocessing of a complete core from the experimental nuclear power plant reactor, it added.

The report added steam charges and the outflow of cooling water at the Yongbyon experimental nuclear power plant had also been observed "consistent with the reactor's operation".

"Since December 2015, when the current operational cycle started, there have been indications consistent with several short periods of reactor shutdown. However, none of these periods were of sufficient duration for the complete reactor core to have been discharged. The Agency's observations indicate that the current operational cycle is longer than the previous one," it said.

It also found "indications consistent with the use of the reported centrifuge enrichment facility located within the plant, including the operation of the cooling units as well as regular movements of vehicles."

North Korea's leader Kim Jong Un and US President Donald Trump held a groundbreaking summit in Singapore in June.

At the meeting the pair struck a vague agreement to denuclearise the Korean peninsula, but there has been little movement since.

Before this, Kim met South Korean President Moon Jae-in in April for their first summit. They agreed to push for a declaration of an end to the Korean War this year.

The two leaders are due to meet again in September.

The IAEA has said previously it stands ready to help verify any future agreements between the US and North Korea over Pyongyang's nuclear weapons programme.