The United States voiced concern Tuesday over human rights in Sri Lanka and urged accountability after allegations that troops summarily killed the 12-year-old son of the Tamil Tiger rebel chief.

A documentary on Britain's Channel 4 -- in a follow-up to explosive footage it aired last year -- alleges that troops executed Velupillai Prabhakaran's son at close range in the bloody finale to the civil war in 2009.

State Department spokeswoman Victoria Nuland said the United States could not authenticate the video but: "As we stated many, many times, we are deeply concerned about allegations of international human rights law and human rights violations in Sri Lanka."

"We support a full accounting of and accountability for anybody who is engaged in acts that violated international human rights and humanitarian law," she told reporters.

The United States is supporting a bid to censure Sri Lanka at the UN Human Rights Council in Geneva, leading the island's ruling party to lead nationwide demonstrations against Washington's position.

Sri Lanka's military has denounced the allegations as spurious and on Monday promised its own documentary to include testimony from frontline troops to clarify events leading to the death of the elder Prabhakaran.

Conflicting accounts of how Prabhakaran was killed in May 2009 suggest he was either gunned down in a shootout or shot while trying to escape advancing Sri Lankan troops in an ambulance.

The entire top military leadership of his Liberation Tigers of Tamil Eelam was wiped out in the final battle, ending a nearly four-decade civil war in which the rebels were known for trademark suicide bombings.

Rights groups have estimated that up to 40,000 civilians were killed in the final offensive, many as the result of military shelling. The government has denied its troops were responsible for any non-combatant deaths.