Many Sri Lankan civilians have been fleeing the conflict zone
Aid workers linked to Sri Lanka's Tamil Tigers have criticised the UN over reports that the rebels are preventing civilians from leaving the war zone.
The UN has said that a growing number of civilians trying to flee have been shot at and some have been killed.
The Tamils Rehabilitation Organisation, widely seen as a rebel front, says the UN has failed to protect civilians.
Tens of thousands of civilians are believed to be trapped after weeks of heavy fighting in the north.
Meanwhile, the United Nations Children's Fund (Unicef) has accused the rebels of stepping up the conscription of child soldiers.
"We have clear indications that the LTTE has intensified forcible recruitment of civilians and that children as young as 14 years old are now being targeted," Philippe Duamelle, Unicef's chief in Sri Lanka, said.
There was no immediate response from the rebels, who have frequently been accused in recent years of using child soldiers.
Most of those civilians caught up in the fighting are believed to be in a government "safe zone" along the north-east 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 TRO is regarded by many as being a rebel front organisation and is banned from operating in the US and UK.
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.
No independent journalists can reach the conflict zone so claims by either side cannot be independently verified.
About 50,000 soldiers are pressing the Tamil Tigers into a patch of north-eastern jungle after taking the key areas of Kilinochchi, Elephant Pass and Mullaitivu.
The government has rejected international calls for a ceasefire, demanding the rebels lay down their arms.
The Tigers have said they will not do so until they have a "guarantee of living with freedom and dignity and sovereignty".