Our correspondnet says the soldiers are being sent to positions about 20km outside around the strategic garrison town of Vavuniya which is some 200km north of the capital Colombo.

BBC's Susannah Price: The aim seems to be to put up new defence lines

The news agency AP reports that some soldiers had abandoned their posts and others had mutinied during the latest fighting which has seen the rebels take control of seral towns in their drive south.

Meanwhile, aid agencies in Sri Lanka have expressed their concern about the plight of civilians and soldiers who surrender, during continuing fighting.

The Tamil Tigers are said to shelling the military base at Omanthai, 10 km from the government-held town of Vavuniya, and there has also been fighting further east.

The International Committee of the Red Cross (ICRC) says it is deeply concerned about the fate of those fighting in the battle, especially soldiers who surrender.

"Past fighting in the country has unfortunately often left many dead and few prisoners," the ICRC said in a statement.

It reminded both the government and the Tigers to comply with international humanitarian law.

French humanitarian agency, Medecins Sans Frontieres (MSF), has also spoken out against the fighting and urged both parties to stop killing civilians.

It also raised concern over transporting supplies into Tiger-held areas.

"We are concerned that should fighting continue for an extended period, the civilian population ... is likely to face severe shortage of food and essential medical supplies," MSF said in a statement.

Goverment setbacks

The Tigers are reported to have taken two more army camps in Welioya, which is further east. Some 7,000 civilians have been displaced.

Navy boats are patrolling the northeast coast in case of a possible attack by the Tigers' naval unit.

Colombo correspondent Susannah Price: "Human rights groups have often expressed their concern about those who surrender"

Major changes were made at the highest levels of the Sri Lankan military following a series of demoralising defeats by the Tamil Tigers last week.

Among those being replaced are the commander of the Wanni area and his colleague in the northern Jaffna peninsula.

The BBC's Susannah Price in Colombo said the Tigers have now regained a large proportion of the land taken by the army during its 19-month offensive called Operation Jayasikuru, or Victory Assured, which was abandoned last December.