North Korea's leader Kim Jong Un must be prosecuted for crimes against humanity, the United Nations human rights envoy said on Monday.

Marzuki Darusman told the UN Human Rights Council that North Korea is devoting huge resources to developing nuclear and other weapons of mass destruction while many of its citizens lack sufficient food and others work in "slave-like conditions".

"We are now at a crucial stage, therefore there is a fundamental need for countries to make that next step in ensuring accountability is undertaken," he said.

Darusman, referring to a report he issued last month, said: "I would like to reiterate my appeal to the international community to move forward to ensure accountability of the senior leadership of the Democratic People's Republic of Korea, including that of Mr Kim Jong Un."

This could be via the International Criminal Court (ICC) but failing consensus among major powers, North Korea's leadership could be prosecuted in a third country, he said.

He called for the Council to set up a panel of three experts to look into "structure and methods of accountability".

Political prison camps, torture, "slave-like labour" and religious persecution remain features of the state apparatus, two years after a landmark UN investigation into crimes against humanity, said Darusman.

The delegation of the Democratic People's Republic of Korea (DPRK) boycotted the session. The European Union, United States and Japan supported Darusman's call for accountability, although they did not refer to Kim by name.

Ambassador Robert King, US envoy on North Korea, denounced the "egregious human rights violations committed by the DPRK" and said that the United States would work with other countries to "seek ways to advance accountability for those most responsible".

However China, an ally of Pyongyang, adopted a more conciliatory tone, saying human rights issues should not be politicised and calling for a comprehensive approach to dealing with North Korea.

China also rejected Darusman's findings that North Koreans who flee across the border to China were being forced back to their homeland illegally.

North Korea Foreign Minister Ri Su Yong, in a speech to the Geneva forum on March 1, said it would boycott any session that examined its record and would "never, ever" be bound by any such resolutions.

Darusan's comments come a month after Christian Solidarity Worldwide (CSW), a religious freedoms charity, called for North Korea to face the ICC for it "egregious human rights abuses".

North Korea conducted its fourth nuclear test in January and launched a long-range missile the following month.

"The denial of human rights to its citizens internally and this aggressive behaviour externally are basically two sides of the same coin. The country is pouring a large amount of resources into developing weapons of mass destruction, while large parts of its population continue to suffer from food insecurity," said Darusman.

Open Doors, the religious persecution charity, rated North Korea the worst place to live as a Christian in its annual report into global discrimination against Christians. The group estimated 70,000 Christians were imprisoned labour camps for their faith.

Additional reporting from Reuters.