COLOMBO (AFP) - Tamil Tiger guerrillas have prevented tens of thousands of civilians from leaving Sri Lanka's war zone and those trying to escape have been "shot and sometimes killed," the United Nations said Monday. The rebels are holding non-combatants in the small patch of coastal jungle in the northeast of the island where they have been cornered by government forces, it added. "A growing number of people trying to leave have been shot and sometimes killed," the UN office here said in a statement, adding the Tigers were also recruiting child soldiers as young as 14 years old. It urged both the Tigers and government forces to find a humane solution so that civilians, including children, could be spared more loss of life due to disease and the fighting. The UN said it welcomed last week's declaration of a larger "safe zone" for civilians along a narrow strip of coastline, but noted there had been fighting even within that area. "This fighting led to the deaths and injury to yet more civilians," the statement said. "The United Nations calls for the Sri Lankan forces and the LTTE (Liberation Tigers of Tamil Eelam) to refrain from fighting in areas of civilian concentration." Some of the wounded were evacuated Monday by the International Committee of the Red Cross (ICRC) which earlier said that a "humanitarian catastrophe" was unfolding in the region where "hundreds" of civilians perished this year. Some 400 injured and their accompanying relatives were shipped out for "further medical treatment," ICRC spokeswoman Sarasi Wijeratne said. The ICRC carried out similar sea evacuations twice last week, bringing 745 wounded from Puttumattalan, within the government-declared safe zone, to the government-controlled northeastern seaport of Trincomalee. The UN said 15 of its local staff and 75 of their dependants were not allowed to leave the war zone by the Tamil Tigers , who had forcibly recruited one of the UN staffers on Sunday. "We are especially concerned that one staff member was reported forcibly recruited into the LTTE yesterday (Sunday)," the UN statement said. "The UN calls on the LTTE to immediately release him, to desist from further recruitment of civilians, and to permit passage for people who wish to leave, especially the women and children." Tens of thousands of civilians in the rebel-held area were experiencing serious shortages of food, medicine, and clean water, the UN said. "Efforts to bring in more food and medicines have not yet been successful, and it is imperative that these needs be met." The UN , the United States and Britain have asked the Tigers to allow civilians to leave the conflict zone while urging the Colombo government to declare a temporary truce. Both have rejected the calls. On Saturday, the defence ministry accused the Tigers of a grenade attack that killed a woman and wounded 13 people who were trying to flee the shrinking area still under rebel control. The Tigers have denied charges that they are attacking civilians and they say the civilians are staying in the area of their own accord. Meanwhile, the defence ministry said sporadic clashes continued in the troubled area where troops on Monday unearthed two dismantled 130-mm long-range artillery guns buried by the rebels before fleeing the village of Visuamadu.