COLOMBO, June 27 (Reuters) - Sri Lanka's government on Wednesday dismissed a claim that evidence in a probe into the massacre of 17 aid workers blamed on security forces was tampered with, and called for a fresh ballistics examination.

The International Commission of Jurists (ICJ) -- an international body of lawyers who have been observing the inquiry -- said this week it suspected a bullet recovered from the corpses of one of the slain aid staff had been substituted.

An Australian forensic pathologist brought in last year to observe post-mortems identified two different kinds of bullet recovered from the remains of the victims,including one shot with a type of weapon mainly used by the military.

But the government's own ballistics expert says there is just one type of bullet commonly used by both sides, accusing the ICJ of being unprofessional and calling the former chief Nordic truce monitor who blamed security forces for the killing a "rogue".

"The ICJ has jumped to the conclusion that there is a strong possibility ... the bullet has been substituted," said Rajiva Wijesinha, head of the government's peace secretariat. "This to me is a clear evidence of a lack of professionalism."

Local staff from international aid group Action Contre la Faim were trapped in the northeastern town of Muttur by fighting between the government and Tamil Tiger rebels in August. Their bodies were found in the group's compound, shot execution-style.

The government has long denied involvement in the killing 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 ICJ has voiced concern that no members of the security forces have been interviewed in connection with the killings nearly a year on. The government says troops have not been interviewed because the truce monitor accusation had put pressure on them.

Human Rights Minister Mahinda Samarasinghe said the government wanted another examination of the ballistics evidence to be carried out by experts from abroad to iron out discrepancies between the reports of the Australian expert and government analyst.

"We have made a request to the Australian High Commissioner for a ballistics expert to be brought so these two experts can sit down and discuss this," Samarasinghe said.

"The government analyst who is the competent authority on this subject in Sri Lanka has come forward with a report. That is the consolidated position of the government."

The aid staff massacre was the worst attack on aid workers since a 2003 bomb attack on the United Nations in Baghdad, and is one of a series of human rights violations levelled at the government as a new chapter in a two-decade civil war escalates.

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