The International Atomic Energy Agency (IAEA) called the continuation of North Korea's nuclear programme "deeply regrettable" in a report, saying it was "a clear violation of relevant UN Security Council resolutions".

"The DPRK's nuclear activities continue to be a cause for serious concern," the Vienna-based agency said, referring to North Korea's official name.

At North Korea's main nuclear complex -- Yongbyon -- construction began in September last year on a new annex and it is now "externally complete", the agency said, adding it could not determine its purpose.

And at a nuclear test site at Punggye-ri, work took place including the reopening of a test tunnel as well as road construction that resumed late last month, the IAEA said in the annual report.

"The reopening of the nuclear test site is deeply troubling, as is the expansion (at Yongbyon) of the reported centrifuge enrichment facility and the continued operation of the 5MW(e) reactor and other facilities," it said.

The report, to be considered by the IAEA General Conference later this month, covers the year until this August.

It incorporates all "relevant information available" to the agency, the IAEA said.

IAEA inspectors were kicked out of North Korea in 2009, and the agency has since monitored it from outside, including through commercial satellite imagery.

In past reports, the IAEA has equally expressed its serious concerns.

North Korea has conducted a record-breaking blitz of weapons tests this year, including firing an intercontinental ballistic missile at full range for the first time since 2017.

Washington and South Korean officials have repeatedly warned that the North is preparing to carry out what would be its seventh nuclear test.

In August, the powerful sister of North Korean leader Kim Jong Un rejected Seoul's offer of economic assistance in return for denuclearisation steps, calling it the "height of absurdity" and a deal Pyongyang would never accept.