COLOMBO, Aug 4 (Reuters) - An Australian expert has rejected allegations of evidence tampering in a probe into a massacre of 17 aid workers blamed on Sri Lankan security forces, the island's government has said.

Australian forensic pathologist Malcolm Dodd last year identified two different kinds of bullet recovered from the remains of the victims, including one shot from a type of weapon mainly used by the military.

However the government's own ballistics expert said just one type of bullet, a 7.62 calibre, was commonly used by both sides in the civil war pitting government forces against Tamil Tiger separatists.

That prompted the International Commission of Jurists -- a body of lawyers who have been observing the inquiry -- to say it suspected the evidence had been tampered with.

"There is no suggestion in my mind of substitution of exhibits," the government quoted Dodd as saying in a statement issued by the human rights ministry overnight.

"All projectiles retrieved from the bodies examined were of the same calibre (7.62)," it quoted Dodd as saying in a supplementary report that has not been published. "A 5.56 calibre projectile can be confidently excluded."

Dodd was not immediately available for comment on the revision of his original findings.

Colombo has denounced as a "rogue" the former chief Nordic truce monitor who blamed security forces for the killings. Yet the case remains unsolved one year after the aid workers were made to lie on the ground and were shot, many in the back of the head, at close range.

Local staff from an international aid group, Action Contre la Faim, were trapped in the northeastern town of Muttur in August 2006 by fighting between government troops and Tiger rebels.

The government has strongly denied involvement in the killings and a presidential commission is probing the deaths. But international observers say the inquiry fails to meet international norms and is heading for failure.

The government has said no troops were interviewed in connection with the killings because the truce monitor's accusation had put pressure on them.

The massacre was the worst attack on aid workers since a 2003 bomb attack on the United Nations office in Baghdad. It is one of a series of human rights violations blamed on the government as a new chapter in the two-decade civil escalates.

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