Sri Lanka’s Tamil Tiger rebels have criticised the UN for saying it had reports that rebels are preventing civilians from leaving the war zone.
The UN has said that a growing number of civilians trying to leave have been shot at and some have been killed.
A rebel front organisation said that the UN had failed to protect civilians.
Tens of thousands of civilians are believed to be trapped after weeks of heavy fighting, mostly in a government "safe zone" along the north coast.
The Tigers have regularly denied claims they are using civilian human shields.
A statement by the Tamils Rehabilitation Organisation (TRO) published on the pro-rebel website TamilNet said: "The UN’s inability to fulfil its obligations to civilians is explicit. Yet they don’t say who is preventing them from their responsibilities."
TRO president Velupillai Sivanadiyar was quoted as saying by the website that the UN was "openly" talking about "withdrawing even the remaining few local staff from the conflict zone, completely shedding its responsibility of caring for the civilians trapped here".
"If they really care for the civilians, this is not the time for useless talk and accusations," Mr Sivanadiyar said.
The UN said on Monday that there are credible reports to suggest that the Tigers are preventing civilians from leaving and a number of those trying to get away are being shot at and in some cases killed.
It said that reports on Sunday indicated that there was fighting inside the government-designated "safe zone."
The UN is calling on both sides to refrain from fighting in areas with large civilian concentrations.
UN spokesman Gordon Weiss also said the Tamil Tigers were trying to forcibly recruit people into their ranks, including children as young as 14.
No independent journalists can reach the conflict zone so claims by either side cannot be independently verified.
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