Many Sri Lankan civilians have been fleeing the conflict zone
The United Nations says it has received reports that Tamil Tiger rebels are actively preventing civilians from leaving Sri Lanka's war zone.
It says a growing number of those trying to leave have been shot at and some have been killed.
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 from the United Nations in Sri Lanka says its concern for the safety of civilians caught up in the fighting has been heightened by reports it has received over the past few days.
Many of the reports are based on testimony from people who have managed to come out of the conflict zone, it says.
It says there are credible reports to suggest that the Tamil Tigers are preventing civilians from leaving and a number of those trying to get away are being shot at and in some cases killed.
Defence Secretary Gothabaya Rajapaksa on claims civilians are being held as human shields
It says 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.
The army has continued to close in on rebel positions
The UN said one staff member had been forcibly recruited into the rebels and demanded his immediate release.
There has been no response from the Tamil Tigers to the latest reports but in the past they have said civilians are frightened and are seeking shelter from the rebels.
They have denied trying to prevent civilians leaving or shooting at them and say the military is constantly firing artillery shells into civilian zones.
On Monday the pro-rebel TamilNet accused the military of using cluster and incendiary bombs on civilian positions.
The government has always denied targeting civilians.
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".