COLOMBO: Sri Lankan troops declared a new safe zone for civilians on Thursday as they battled to finish off the island's drawn-out ethnic conflict with separatist Tamil Tiger rebels, the defence ministry said.

Concern has mounted for tens of thousands of non-combatants trapped in the war zone, with the International Committee of the Red Cross (ICRC) saying hundreds have already been killed.

Britain named former defence minister Des Browne as a special envoy to the war-scarred island nation, amid mounting international concern about the fate of civilians, but Colombo quickly rejected the move, a top official said.

London said Browne was to focus on the "immediate humanitarian situation in northern Sri Lanka", but the cabinet of Sri Lankan President Mahinda Rajapakse said the envoy would not be accepted here, the official told AFP.

The government has asked men, women and children to move to a 12-kilometre stretch of coastline as troops advanced on rebel positions in the north in a bid to crush all remaining pockets of Tiger resistance.

In creating the new safe zone, the government effectively scrapped a 35-square-kilometre designated no-fire area.

"The Sri Lanka army, fully committed to provide maximum safety for the lives of entrapped or forcibly detained civilians... calls upon the public to move into those specified areas at the earliest," the defence ministry said.

Military officials said there was heavy fighting in the area where some 700 Tiger rebels were believed to be offering stiff resistance. No details of casualties from Thursday's clashes were released by either side.

Security forces seized nine mortar tubes from a fortified camp at Kuppilankulam on Wednesday, the ministry said in a statement, adding 28 Tamil Tiger guerrillas had been killed in fighting in the area.

Also seized were a workshop for making hand grenades and roadside bombs, and several vehicles including an armour-plated van used by Tiger leader Velupillai Prabhakaran, the ministry said.

Military officials have speculated that Prabhakaran - who launched the Tigers' quest for an independent homeland nearly 40 years ago - may have slipped out of the country by boat as his fighters have been driven back.

The government has said it is on the brink of crushing the rebels.

The defence ministry said the ICRC had been informed of the creation of the new safe zone, where the international relief agency already operates a make-shift medical facility.

An ICRC official said they had carried out the evacuation on Thursday of 160 patients from a makeshift hospital in Putumattalan where artillery shelling killed 16 people earlier in the week.

Spokeswoman Sarasi Wijeratne said the patients were taken by ferry to the eastern port of Trincomalee for treatment and that they were accompanied by three international Red Cross staffers.

Only 130 local ICRC employees remain in the conflict zone.

The Red Cross carried out a sea evacuation of 240 patients on Tuesday, saying the group had been stranded at the makeshift hospital deep inside rebel territory for more than a week.

The aid organisation said 16 patients died when shells hit the Putumattalan facility on Monday, but it did not say who was responsible for the attack.

Claims by either side cannot be verified as human rights groups, diplomats and independent journalists are not allowed to report freely from the conflict zone.