The UN human rights chief Wednesday called for an International Criminal Court investigation into Sri Lanka's Tamil separatist conflict and sanctions against top generals and others accused of war crimes.

Michelle Bachelet accused Sri Lanka of reneging on promises to ensure justice for thousands of civilians killed in the final stages of the 37-year separatist war that ended a decade ago.

"Domestic initiatives for accountability and reconciliation have repeatedly failed to produce results, more deeply entrenching impunity, and exacerbating victims' distrust in the system," she said in her latest report on Sri Lanka.

The government of President Gotabaya Rajapaksa has reversed advances made under previous administrations and the South Asian nation was on an "alarming path towards recurrence of grave rights violations," according to the report .

President Gotabaya Rajapaksa (right) was the top defence official when government forces crushed the guerrillas. His brother Mahinda (left) was president then and is currently prime minister / © AFP/File

Surveillance of rights activists and dissidents has increased and a climate of self-censorship has emerged, it added.

Rajapaksa won a 2019 presidential election on a nationalist agenda which included a promise that troops who crushed Tamil rebels would not be prosecuted.

He was the top defence official when government forces crushed the guerrillas in a military campaign that ended in May 2009. His brother Mahinda was president then and is currently prime minister.

- 'Asset freezes' -

UN reports have accused Sri Lankan troops of shelling hospitals and indiscriminate aerial bombardments, executing surrendering rebels and causing the disappearance of thousands of minority Tamils.

At least 100,000 people were killed in the war and allegations were made that 40,000 Tamil civilians were killed in the final onslaught.

The president, a retired army lieutenant colonel, threatened last year to withdraw from the UN rights council if it pursued allegations against his troops.

In her latest assessment, Bachelet recommended for the first time that the ICC look into Sri Lanka's case, and said action should be taken against war criminals, including members of the defeated Tamil Tiger guerrilla group.

"Member states can actively pursue investigation and prosecution of international crimes committed by all parties in Sri Lanka before their own national courts," she said.

The 16-page report to be formally presented to the UN Human Rights Council on February 24 also calls for possible targeted sanctions "such as asset freezes and travel bans against credibly alleged perpetrators" of rights violations.

Sri Lanka had responded to the report, but Colombo's reaction was not immediately made known.

In the damning report, Bachelet expressed concern at General Shavendra Silva's elevation as army chief and General Kamal Gunaratne's appointment as defence secretary. UN reports have implicated both in alleged war crimes, she noted.

Silva, who was a field commander at the height of the separatist war, already faces a US travel ban.

Sri Lanka has resisted repeated calls for an independent investigation and the Rajapaksa brothers had previously denied any war crimes were committed.

However, ahead of next month's UN Human Rights Council sessions in Geneva, President Rajapaksa last week did a U-turn and said Sri Lanka would investigate some allegations.

He gave a commission of inquiry six months to look into previous inquiries into allegations of "human rights violations, serious violations of international humanitarian law".

However, the UN rights body noted Wednesday that the new commission "lacks diversity and independence, and its terms of reference do not inspire confidence it will produce any meaningful result."

The UN rights body said that, since returning to power, the president had undermined previous police investigations and may have contributed to the destruction of evidence.

Bachelet called on member states to take action to preserve evidence from key cases such as the killing of 17 aid workers from a French charity in August 2006 and the 2009 assassination of newspaper editor Lasantha Wickrematunge.

The UN rights chief said several top police officers involved in high-profile cases had been penalised or arrested to stifle investigations.

She also criticised President Rajapaksa for granting a pardon in 2020 to an army officer convicted and jailed for killing eight Tamil civilians, including four children, in April 2000.