Sri Lanka has rejected a fresh appeal by the UN to give civilians more time to leave a safe zone in the north-east, the defence secretary says.
Only a few hundred civilians used the two-day unilateral government truce with Tamil Tiger rebels to escape.
Between 50,000 and 100,000 civilians are still thought to be trapped in the zone – about 20 sq km (8 sq miles) of coastal area in Mullaitivu district.
Security forces are in what they say is a final push to defeat the rebels.
The government’s decision to reject the UN appeal came after a meeting between Vijay Nambiar, chief of staff of UN Secretary-General Ban Ki-moon and senior government officials in Colombo this week.
"I told him (Mr Nambiar) that we cannot extend our decision to restrict offensive military operations because there was no result during the previous halt in the fighting," the Sri Lankan defence secretary, Gotabhaya Rajapakse, said in an interview with the BBC.
The Tamil Tigers did not meet international demands to release the civilians during the pause in the fighting that ended on Tuesday night, he said.
Mr Ban sent Vijay Nambiar, to Sri Lanka on Thursday to discuss the fate of the civilians and efforts to free them from the war zone
But Tamil Tigers had said the temporary ceasefire was an "act of hoodwink" designed to deceive the international community. They said there should be an internationally supervised truce, paving way for political solutions.
The Sri Lankan military says after months of intense battles the rebels have now been pushed into the no-fire zone and troops have now surrounded the area on three sides.
Aid agencies have already warned that there could be a bloodbath on the beaches of north-eastern Sri Lanka if the confrontation is taken inside the safe zone.
‘Careful monitoring’
Mr Rajapakse, the most senior civilian official in charge of the war, said government forces will not launch any massive military assault on the safe zone due to the presence of the civilians.
"It’s a hostage rescue operation, something like the Entebbe rescue mission," he said, referring to Israel’s 1976 rescue of more than 100 hostages from Entebbe airport in Uganda.
"It has to be discreet and surgical.
"We are carefully monitoring the ground situation. When the time comes we will go in, it is purely for the field commanders to decide."
At the same time, Mr Rajapakse said, the government did not want a prolonged operation because that will give more time for the rebels to regroup.
There has been no immediate reaction from Tamil Tigers on the defence secretary’s comments, but in the past they have denied allegations that they were holding civilians as hostages.
They say the civilians do not want to leave the no-fire zone because they fear the military.
More than 65,000 civilians have already fled the war zone in recent months and they have been housed in government-run relief centres and camps in the northern region.
Mr Rajapakse once again denied allegations that security forces were firing heavy weapons in the safe zone.
"However, if the rebel leader, Prabhakaran, is seen then we will take him out, even if it is inside the safe zone. But we will take all measures to avoid civilian casualties," he said.
The UN says more than 2,800 civilians may have been killed and 7,000 others injured in the fighting in the north-east in the last two months. The government disputes these figures.
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