The United Nations has decried Sri Lanka's "worryingly slow" progress in addressing wartime crimes, warning that a range of serious abuses, including torture, appear to remain widespread.

The world body and rights activists have previously accused the Sri Lankan military of killing thousands of civilians, mostly Tamils, in the last weeks of a 26-year civil war with Tamil separatists that ended in 2009 and have demanded redress.

In its latest report released on Friday, the UN expressed alarm that Sri Lankan security and police forces are reportedly still committing rape and torture, and said the country must investigate and prosecute those involved, as well as those implicated in past wartime atrocities.

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"Cases of excessive use of force, torture, arbitrary arrests and failure to respect due process during arrests by the police also continue to be reported," it added.

The lack of accountability risks derailing the momentum towards lasting peace, said the report by the office of UN High Commissioner for Human Rights (OHCHR) Zeid Ra'ad al-Hussein.

"I urge the government and people of Sri Lanka to prioritise justice alongside reconciliation to ensure that the horrors of the past are firmly dealt with, never to recur," Zeid said.

"The authorities at all levels, from the head of state to military, police, intelligence and local-level leaders, need to publicly issue unequivocal instructions to all branches of the military, intelligence and police forces that torture, sexual violence and other human rights violations are unequivocally prohibited and will be punished," Zeid said.

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"OHCHR received credible information from a well-known human rights organisation according to which 'white van' abductions, and torture and sexual violence, by the Sri Lankan security forces persisted. These allegations must be properly investigated," the report said.

The UN and activists have also demanded that Sri Lanka include foreign judges in a credible inquiry, which President Maithripala Sirisena has opposed.

A task force appointed by Prime Minister Ranil Wickremesinghe, however, said in January the country should bring in international prosecutors and judges.

Addressing the UN Human Rights Council in Geneva on Tuesday, Foreign Minister Mangala Samaraweera said the government "maintains a zero-tolerance policy towards torture".

He said he expected draft legislation on a truth-seeking commission to be presented to the cabinet "within the next two months".