Amnesty International has accused Sri Lanka of illegally holding hundreds of detainees who are vulnerable to torture and execution and urged the UN to investigate allegations of serious abuses during and after the country's 26-year civil war.

In a new report entitled "Locked away: Sri Lanka's security detainees", the rights group also said arbitrary and illegal detentions and enforced disappearances were routine in Sri Lanka, where rights abuses "go uninvestigated and unpunished."

The lack of accountability for alleged war crimes committed by the government side has created a sense of impunity among security forces, the report released on Tuesday said.

Most of the detainees were arrested under counter-terrorism laws introduced during Sri Lanka's long struggle against the separatist Liberation Tigers of Tamil Eelam (LTTE), who were defeated after a massive military offensive in May 2009.

'Horrific abuse'

Sri Lanka's Foreign Minister Gamini Lakshman Peiris recently said there were 750 Tamil Tiger ex-combatants still in detention.

“The LTTE had a horrific record of abuse, including killing and imprisoning its critics, but that did not, and does not, excuse the widespread and systematic mistreatment of detainees by the Sri Lankan government," Sam Zarifi, Amnesty's director for Asia Pacific, told reporters in Geneva.

The UN's Human Rights Council is expected to vote next week on a US-backed resolution calling for accountability in Sri Lanka.

A television documentary to be aired by Britain's Channel 4 on Wednesday alleges several war crimes suspects are now in senior government posts.