Colombo - A visiting United Nations envoy to Sri Lanka expressed serious concern Thursday about civilians in the northern part of the country who have become trapped in the fighting between government troops and Tamil rebels.

UN Under-Secretary General on Humanitarian Affairs John Holmes opening a two-day visit to Sri Lanka by calling on Tamil rebels to permit civilians to move freely and allow them to leave the areas of fighting.

Holmes called on both the government and the rebels to ensure the safety of civilians and said that the UN will help provide humanitarian assistance to people trapped in the rebel-held areas and those who have fled to government-controlled areas.

The UN envoy held talks with Foreign Minister Rohitha Bogollagama and will visit welfare centres where some 30,000 displaced persons who fled the rebel-controlled areas are being accommodated.

These displaced persons are in Vavuniya, 240 kilometres north of the capital.

The visit comes as fighting continues in a part of the Mullaititvu district, 390 kilometres north-east of the capital, where the rebels are reported to be confined to an area of 100 square kilometres.

The government estimates that at least 70,000 people are trapped in this area and claims that the rebels of the Liberation Tigers of Tamil Eelam (LTTE) were not allowing them to leave.

The pro-rebel website Tamilnet said that at least 50 civilians were killed and 70 injured in an airstrike by the air force in the Mullaitivu area, a claim denied by the military.

There has been no independent confirmation about the casualty figures provided by the rebels, but the International Committee of the Red Cross (ICRC) has confirmed large numbers of casualties without blaming any party as responsible for the attacks.

The pro-rebel websites provided pictures purported to be taken of those injured and killed in the conflict.

Government troops launched their military offensive to recapture rebel-held areas in the north and east in August 2006 and since then have captured large areas held by the rebels and say they are now in the final phase of crushing the rebels. (dpa)