North Korea has accused the UN agency responsible for regulating atomic energy of being a puppet of hostile countries after a new report said the isolated nation's nuclear weapons stockpile was breaking international law.

Pyongyang has gradually built an atomic stockpile after abandoning the Non-Proliferation Treaty in 2003, and has tested several nuclear bombs in the years since.

Since Kim Jong Un took over from his father as the country's supreme leader, North Korea's military has made rapid strides in its nuclear weapons and ballistic missile programmes, and has been subjected to increasingly strict international sanctions as a result.

The International Atomic Energy Agency, whose inspectors have not been allowed into the country for more than a decade, said Wednesday that Pyongyang's weapons programme was "deeply regrettable."

North Korea's nuclear activities "remain a cause for serious concern," said agency chief Rafael Mariano Grossi in a report to the UN General Assembly.

Pyongyang's ambassador to the UN hit back later in the same forum, telling other diplomats that the IAEA's report was "grossly distorted."

"The IAEA is no more than a political tool of the Western countries," he said, adding that the body was "a marionette dancing to the tune of the hostile forces" against North Korea. 

Nuclear talks between North Korea and the US have been deadlocked since the collapse of a summit between Kim and President Donald Trump in Hanoi early last year.

North Korea is widely believed to have continued to develop its arsenal -- which it says it needs to protect itself from a US invasion -- throughout the discussions.