The United Nations plans to make a second attempt to evacuate hundreds of critically wounded civilians, including at least 50 children, from war-torn northern Sri Lanka [Images].

The UN says that if permission is granted by the Liberation Tigers of Tamil Eelam, and if a lull in fighting permits, the convoy will cross the frontline on Thursday noon, UN Secretary General Ban Ki-moon's spokesperson said.

The injured will then be transported to government hospitals in the town of Vavuniya that can cope with their wounds, he added.

The convoy evacuating the civilians has been trapped for days in the town of Puthukkudiyiruppu, which lies just across the lines of confrontation in territory controlled by the LTTE [Images].

An estimated 250,000 civilians are trapped in areas of northern Sri Lanka where fighting continues between government forces and the LTTE.

Nearly 5,000 people have managed to cross the zones held by the group to government-controlled areas since late November, UN Office for the Coordination of Humanitarian Affairs says.

Ban, has called for all parties to allow and facilitate the movement of the 250,000 civilians currently caught up in fighting to safe areas.

"He calls on the LTTE and the Government of Sri Lanka to accord immediate and absolute priority to ensuring the protection and well-being of civilians, including humanitarian aid workers," his spokesperson said in a statement.

Ban also called on both parties to respect "no fire zones," "safe areas," and civilian infrastructure including schools, medical facilities and humanitarian facilities and assets.