"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 the director general of 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 duration of the steam plant's operation was not sufficient to have supported the reprocessing of a complete core from the experimental nuclear power plant reactor, it added.

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.