Sri Lanka could be hauled before a war crimes tribunal over the killing of "many thousands of civilians" in the final months of its separatist war with Tamil rebels, a top US official said.

In the toughest warning since the end of fighting in May 2009, US Assistant Secretary of State for South and Central Asian Affairs, Robert Blake, said Sri Lanka risked a forced international investigation.

His remarks came as the UN Human Rights Council was meeting in Geneva, where Sri Lanka has previously managed to avoid condemnation thanks to the backing of several nations, including strong allies China and Russia.

Blake, a former ambassador to Sri Lanka, said it was "preferable" for Sri Lanka to have its own investigation in line with internationally accepted human rights standards, rather than face an external inquiry.

"It's important to say that if Sri Lanka is not willing to meet international standards regarding these matters, there would be pressure to appoint an international commission to look into these things," Blake told AFP.

Sri Lanka has refused to investigate alleged war crimes, but President Mahinda Rajapakse has appointed a panel to probe why a 2002 ceasefire between the government and the Tamil rebels broke down.

International rights groups have rejected the government-appointed panel as a whitewash, saying it fails to address war crimes by both sides during the 37-year conflict.

Sri Lanka's relations with the US were strained in 2009 when Washington voiced concern about human rights as the army killed the top leadership of the Tamil Tiger rebels, ending decades of insurgency.

After urging from the Tamil diaspora, a number of US lawmakers have pushed President Barack Obama's administration to take a harder line on Sri Lanka.

In a video interview on Monday from Washington, Blake said reconciliation, accountability and human rights were among the key elements the US wanted to see to normalise ties with the South Asian nation.

"The UN has estimated that many thousands of civilians were killed in the final few months of the war," Blake said.

The UN said that at least 7,000 civilians perished in the final months of fighting, while international rights groups have put the toll at more than 30,000.

Blake cited the UN Security Council reporting Libya to the International Criminal Court last week as a signal of global concern over human rights.

He said he was not comparing Sri Lanka with Libya, but said the security council's unanimous decision against Libya over the weekend underscored the resolve of the international community regarding crimes against humanity.

(Soure: AFP)