Nordic truce monitors on Thursday accused both the government and the Liberation Tigers of Tamil Eelam (LTTE) of grossly violating the terms of a now tattered 2002 ceasefire, saying both sides were hampering investigations of rights abuses.

"I am deeply disturbed by the report of the Sri Lanka Monitoring Mission that over 200 civilians have been killed, and thousands displaced, over the past two months in Sri Lanka," said Eric Schwartz, deputy United Nations special envoy for tsunami recovery, in a statement issued overnight.

Many foreign aid groups have had to abandon post-tsunami projects in the northeast amid the worst outbreak of fighting since the truce, which has killed hundreds of civilians, troops and rebels.

"Significant investments in reconstruction, so generously supported by donors around the world, are now imperilled," said Schwartz, deputy to the special UN envoy, former US President Bill Clinton. "More importantly, many thousands of civilians are at grave risk, cut off from regular supplies of food and other assistance."

"It is critical that all parties to the conflict ease current restrictions on access to affected populations, cease hostilities and return to negotiations," he added.

The government and the Tigers insist they honour the truce and accuse each other of trying to force a full-blown return to a war that has killed more than 65,000 people since 1983.

And while both sides have pledged to eventually resume peace talks that have been suspended for five months, analysts and diplomats fear violence could well deepen first.