Harsh human rights conditions in North Korea have barely changed and its leader, Kim Jong-un, should be held criminally responsible, the top UN envoy on North Korea said Friday.

Marzuki Darusman said in Tokyo that his repeated requests to visit North Korea during six years as the UN special rapporteur have never been accepted.

He said global efforts to improve human rights in North Korea, officially called the Democratic People's Republic of Korea, must continue.

"In addition to continuing political pressure to exhort the DPRK to improve human rights, it is also now imperative to pursue criminal responsibility of the DPRK leadership," he said.

He said various institutions, civil groups and governments, as well as the UN are collecting information "to identify the perpetrators, and to trace all these actions (of abuse) to the highest leadership in the country." Judicial proceedings have not started, and he declined to identify the perpetrators.

"There is a need to build up a strong case so that accountability is not compromised," he said. "When the moment comes, the judicial processes are made possible."

Darusman was in Japan to assess North Korean human rights developments. He talked with Japanese police and legal experts, as well as relatives of Japanese citizens abducted by North Korea decades ago. His visit follows a North Korean nuclear test earlier this month.

In 2014, a UN commission of inquiry on North Korean human rights issued a report detailing starvation, torture, arbitrary arrests, imprisonment and executions. Darusman said little has changed since then.

North Korean Ambassador-at-large Ri Hung Sik said in November that he had met Darusman once, but "we don't see any benefits" in talking to him again because "he has been talking of regime change whenever he's abroad."