By Simon Gardner

COLOMBO, Aug 6 (Reuters) - Sri Lanka's government is responsible for unlawful killings and disappearances, Human Rights Watch said on Monday -- the anniversary of the discovery of the massacre of 17 aid workers blamed on security forces.

Issuing a report entitled 'Return to war: Human rights under siege', the U.S.-based group said President Mahinda Rajapaksa's government is resorting to abuses to fight a new chapter in a two-decade civil war against Tamil Tiger rebels.

"The Sri Lankan government has apparently given its security forces a green light to use 'dirty war' tactics," Brad Adams, Asia director at Human Rights Watch, said in a statement.

"Abuses by the LTTE (Tigers) are no excuse for the government's campaign of killings, disappearances and forced returns of the displaced," he added. "The government has repeatedly promised to end and investigate abuses, but has shown a lack of political will to take effective steps."

About 70,000 people have been killed in the conflict between Liberation Tigers of Tamil Eelam, fighting for a separate state for minority ethnic Tamils, and security forces since 1983.

Rights groups say hundreds of people, many of them minority Tamils, have been reported abducted or disappeared this year and 1,000 more in 2006. Rebels, paramilitaries, elements of the security forces, and underworld gangs have all been blamed.

The Sri Lankan government says numbers of disappearances are overblown and many cases are fakes to discredit the administration.

The publication of the Human Rights report coincides with the commemoration of the murder of 17 local staff members of Paris-based aid group Action Contre la Faim, who were shot dead in their compound in the northeastern town of Muttur last August after they were trapped by fighting between troops and rebels.

Nordic truce monitors blamed the killings on the security forces and international observers say an inquiry into the massacre, the worst attack on aid workers since the 2003 bomb attack on the United Nations office in Baghdad, fails to meet international standards.

Action Contre La Faim are not pointing fingers, and are waiting for answers in a case that remains unsolved a year on.

"We want to know who has done this," said Loan Tran-Thanh, head of the group's Sri Lanka mission. "It's very slow, but that's normal."

"We cannot make any judgements ... because we are not the experts. We don't have enough data for us even to give an opinion," she added. "There have been so many contradictions."

The island's human rights minister demanded that journalists be barred from the commemoration ceremony.

The Tigers are also blamed for serial abuses, including killing civilians and troops with roadside bombs and forcibly recruiting people, including children, to fight in the war.

Human Rights Watch is lobbying for a United Nations human rights mission to be sent to Sri Lanka in the name of transparency and to discourage further abuses, but the government has refused.

It says western governments are bullying it on human rights and are hypocritical, citing abuses in Iraq and Afghanistan.

"I understand that they are going to commemorate ... these 17 people, but they have forgotten 35 people from the Muslim community butchered in the same place by the LTTE," said government defence spokesman Keheliya Rambukwella.

"How do these human rights work with 17 and not work for 35?" he added. "As far as the government is concerned, it is doing everything possible in relation to human rights." (Additional reporting by Ranga Sirilal in COLOMBO)

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