Nordic truce monitors on Wednesday formally accused the security forces of being behind the execution-style murders of 17 local staff of aid agency Action Contre La Faim earlier this month in the northeast.

The government denies it, and angry officials accused the monitors of being biased toward the rebels in their ruling.

"We have no independent information ourselves in the UN, but I say we cannot continue in this area unless people will be held accountable for the execution of 17 of our colleagues," the UN Emergency Relief Coordinator, Jan Egeland, told reporters in New York.

The victims, mostly Tamils, were found shot dead in their compound in the northeastern town of Mutur, around 200km northeast of the capital Colombo. It was the worst mass murder of aid staff since a 2003 bomb attack on the United Nations compound in Baghdad.

The Nordic Sri Lanka Monitoring Mission (SLMM), which oversees a 2002 truce that now only holds on paper, says Sri Lankan authorities have obstructed their efforts to investigate, and says it is convinced no armed groups other than the security services could have been responsible.

"I will have some problems to trust a government investigation now because they are too involved in this case," outgoing chief monitor Maj. Gen. Ulf Henricsson said in an interview.

"A democratic and accountable government should support an international commission to look into this case."

"This is not just a Sri Lankan problem. This is a worldwide problem if you can kill aid workers without any punishment."

Meanwhile, the Sri Lankan government demanded proof yesterday of allegations by Nordic truce monitors that security forces were responsible for the massacre of 17 aid workers of a French charity.

"It is premature to blame the government without any supporting evidence," Sri Lankan government spokesman Keheliya Rambukwella said in Colombo.

The Sri Lanka Monitoring Mission (SLMM) said Wednesday that Sri Lankan troops were involved in the execution-style slayings of the 17 local workers of Action Against Hunger during battles with Tiger Tamil rebels near the town of Muttur in the northeast earlier this month.