* Sri Lankan envoy dismisses report
* Rapes punishments, to extract confessions - report
* UK urged to review asylum for Sri Lankans at risk (Adds call for UK to review asylum policy)
By Nita Bhalla
NEW DELHI, Feb 26 (Reuters) - Sri Lanka's security forces have used rape to torture and extract confessions from suspected Tamil separatists almost four years after the country's civil war ended, Human Rights Watch (HRW) said in a report on Tuesday.
The rights group documented 75 cases of predominately Tamil men and women who said they were held in Sri Lankan detention centres and repeatedly raped and sexually abused by the military, police and intelligence officials.
The victims - now living as asylum seekers, most of them in Britain - said once they confessed to being a member of the Tamil Tiger rebel group, the abuse generally stopped and they were allowed to escape by paying a bribe.
"We found that rape was used to secure some sort of confession, but also as a political tool to punish people," Meenakshi Ganguly, the rights group's South Asia director, told a news conference in New Delhi.
Other victims said they were "severely tortured, burnt by cigarettes and hung upside down," he added.
Sri Lanka's High Commissioner to New Delhi, Prasad Kariyawasam, said he had no evidence to suggest the allegations of abuse, which the rights group said occurred from 2006 to 2012, were true.
The envoy said the testimonies of 41 women, 31 men and three boys were likely made by "economic refugees" who "need a good story" to get asylum.
"Until we do a proper inquiry, we have to believe that these are all sob stories for the sake of obtaining asylum or refugee status in a developed country," Kariyawasam told Reuters.
He said the report was "a well-timed effort" to discredit Sri Lanka ahead of a vote on a U.S.-backed resolution criticising it at the U.N. Human Rights Council in Geneva this week.
Tens of thousands of civilians were killed in 2009, in the final months of a war that began in 1983, a U.N. panel said, as government troops advanced on the last stronghold of the rebels fighting for an independent homeland.
The U.N. panel said it had "credible allegations" that Sri Lankan troops and the Tamil Tigers both carried out atrocities and war crimes, and singled out the government for most of the responsibility for the deaths.
Sri Lanka has come under international pressure to bring to book those accused of war crimes and boost efforts to reconcile a polarised country.
It has rejected allegations of rights abuse and resisted pressure to allow an independent commission to investigate war crimes committed by its army, saying that it is has its own plan to deal with the issue.
But Human Rights Watch said, despite the end of the war, no one had been prosecuted and human rights violations of Tamil Tiger supporters continued. Thirty-one cases of rape and torture in the report had been documented since 2009.
"Many of the medical reports examined by HRW show evidence of sexual violence such as bites on the buttocks and breasts, and cigarette burns on sensitive areas like inner thighs and breasts," the group said.
At the Geneva meeting, the United States is expected to sponsor a resolution for the second time censuring Colombo and urging it to prosecute soldiers suspected of killing civilians.
Britain, Canada and the European Union, where there is a large presence of Tamil refugees and asylum seekers, are expected to support the resolution.
Human Rights Watch also said Britain needed to review its asylum policy to ensure that people who are forcibly sent back to Sri Lanka are not at risk of torture and ill-treatment on their return.
The group's UK director, David Mephan, told Reuters a flight carrying 60 Sri Lankan nationals was scheduled to leave Britain for Sri Lanka on Thursday and there was a chance some of them might be at risk.
The UK Border Agency defended its systems saying people who could show they were at risk of being tortured were offered protection. (Additional reporting by Maria Caspani in London; Editing by John Chalmers and Robert Birsel)