UN human rights chief Navi Pillay has called for an international investigation into what she said were decades of violations in North Korea, including torture and executions of political prisoners.

She voiced regret there had been no improvement since Kim Jong-un took power a year ago, and said it was time for the international community to help bring change for North Korea's "beleaguered, subjugated population".

She said: "An in-depth inquiry into one of the worst – but least understood and reported – human rights situations in the world is not only fully justified, but long overdue."

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Japan is considering whether to submit a resolution on North Korea to the UN Human Rights Council (UNHCR), diplomats said.

UNHCR special investigators have been denied entry into North Korea for years. However, the council has the power to launch wider independent international inquiries which can build a criminal case.

Ms Pillay regretted that concerns about North Korea's nuclear programme and rocket launches were overshadowing "the deplorable human rights situation in the DPRK [Democratic People's Republic of Korea] which in one way or another affects almost the entire population and has no parallel anywhere else in the world".

Former New Mexico state governor Bill Richardson and Google executive chairman Eric Schmidt failed to secure the release of a Korean-American held in North Korea during a trip to the state last week.

The reclusive country's network of political prison camps is believed to contain 200,000 people or more and have been the scene of widespread violations including rapes, torture, executions and slave labour, according to Ms Pillay, a former judge at the International Criminal Court.

Living conditions in the camps are atrocious with insufficient food, little or no medical care and poor clothing, and the threat of death penalty for minor crimes, she added.