The Tamil Tigers are being restricted to a shrinking jungle area
The United Nations has accused Sri Lanka's Tamil Tiger rebels of forcibly recruiting a UN staff worker along with members of his family.
The UN said the man and three members of his family, among them his 16-year-old daughter, were taken.
The UN said it feared for the safety of the four, who were seized over the weekend - the second such incident it has reported in the past two weeks.
The Tigers have made no comment yet on the UN statement.
However, the BBC's Anbarasan Ethirajan in Colombo says they have in the past always denied forcible recruitment.
The UN statement comes as tens of thousands of civilians remain trapped in the conflict zone in the country's north-east.
A military operation over the past few months has pushed the rebels back into a small area of jungle where there is heavy fighting.
The military says the rebel area has now been reduced to 30 sq km (12 sq miles).
The UN statement said: "The UN in Sri Lanka has protested to the [Liberation Tigers of Tamil Eelam] that UN national staff, as well as children in general, are protected under national and international law from recruitment by armed groups, and has called for their immediate release."
Another UN national staff member recruited two weeks ago has yet to be released, it said.
Last month, the agency accused the rebels of holding 11 of its local employees and more than 50 of their family members in the conflict zone against their will.
In another incident, UN spokesman Gordon Weiss said on Monday that "the wife of a UN staff member was injured by an anti-personnel mine while escaping with the staff member and their two children".
The organisation says some 2,800 civilians have been killed in the fighting this year. The government says those numbers are inflated and unsubstantiated.
The government has designated a "safe zone" for civilians but military spokesman Brig Udaya Nanayakkara said troops were "closing into the safe zone in certain areas".
The government says hundreds of civilians are still fleeing the war zone.
The UN and international powers have called for a ceasefire to allow civilians to be evacuated but the government says that will only allow the rebels to regroup.
Pro-rebel sources say the army is targeting civilians.
The Tigers' political chief B Nadesan told the TamilNet website the government was committing war crimes and crimes against humanity.
Independent reporters are not allowed in the war zone and information from either side cannot be verified.