Retired Australian judge Michael Kirby, who headed a UN inquiry into rights violations in North Korea, says Pyongyang's actions, while constituting a crime against humanity, fell short of genocide.

On Tuesday he reiterated the panel's findings last year that "crimes against humanity have been committed" by the North Korean government in its mistreatment of thousands of prison detainees.

"This is a very serious finding," said Kirby, speaking to reporters in Washington.

"It imposed on the international community to make those who are responsible accountable," he said, adding that the UN human rights body has made North Korea's leader aware of their findings.

The UN inquiry, which wrapped up last year, found that North Korea's human rights violations were "without parallel in the contemporary world."

The chairman said he was disappointed that the panel was constrained by a "very narrow definition" of what constitutes genocide as they drafted their findings.

"It is a 1948 definition and it was not wide enough for us to find genocide and we did not."

The panel nevertheless urged the UN Security Council to refer Pyongyang to the International Criminal Court (ICC), and made its finding known to North Korean leader Kim Jong-Un.

"We wrote a letter to the Supreme leader warning him that the officials of his government and possibly he himself might be accountable," Kirby said, in remarks made at the Centre For Strategic and International Studies, a Washington think tank.

The council, following the report, also convened its first-ever meeting on Pyongyang's rights record, which was held despite opposition from China.

The investigation, whose findings were released a year ago, gathered testimony from 300 witnesses and corroborating evidence that documented a vast network of prison camps believed to hold as many as 120,000 people.

The report said atrocities carried out at the camp include torture, rape and summary executions.

Kirby expressed disappointment with some aspects of the final report, including the limited contact with officials from North Korea.

"We tried in every possible way... to engage with them, but they wouldn't engage with us except on very limited terms favourable to them," he said, repeating his call for the case to be brought before the ICC.