COLOMBO (AFP) – Sri Lanka's advance against Tamil Tiger rebels, now in its final stages, has been slowed considerably by the presence of civilians in the war zone, the nation's top defence official says.

An estimated 70,000 civilians are inside the shrinking territory in the coastal area of Mullaittivu, into which the rebels have been penned after losing their mini-state, Defence Secretary Gotabhaya Rajapakse said.

"The military is taking more casualties now because they can no longer soften the target using artillery and air attacks," he told AFP late Tuesday in an interview at his tightly guarded sea-front office in the capital. 

At the same time, he said troops were on the verge of victory after 37 years of fighting the Liberation Tigers of Tamil Eelam (LTTE), who have been struggling for a separate Tamil state within the Sinhalese-majority island. 

The plight of non-combatants has sparked UN-led calls for a truce to allow civilians to leave. The International Red Cross warned earlier this month a humanitarian catastrophe was unfolding in the region. 

But Rajapakse, who is President Mahinda Rajapakse's younger brother, said a ceasefire was unnecessary as troops were not attacking a coastal stretch designated a no-fire zone in order to allow civilians safe passage. 

"There is no meaning to (having) a ceasefire now," said Rajapakse. 

Government officials say some 35,000 civilians have crossed the front lines despite the rebels firing on fleeing men, women and children. The UN and other governments have also accused the Tigers of attacking escaping civilians. 

But the Tigers have strongly denied the allegations and say the civilians are remaining in the territory on their own accord. 

Rajapakse said the length of the hostilities depended "on how quickly the Tigers release civilians." 

"If there are no civilians in that area, that will also mean the end of the LTTE," he said, accusing the rebels of using the civilians "as a human shield." "They also take part of the food and medicines sent for the civilians. Without civilians they have no supply line," he said. 

Rajapakse, a retired army colonel and main figure spearheading the campaign, said the first stage of military operations would end after all rebel territory was seized. 

"I would not say we have defeated the Tigers completely until we have completed all three phases of our operation," he said, adding the next would be to mop up remaining resistance and seize all guerrilla weaponry. 

The final phase would be to ensure stability. 

"We're not going to leave any room for them to come back," he said. 

After the Tigers quit peace talks in 2003 and refused to return to the negotiating table, the government almost doubled the military's strength and increased its weaponry. 

The battlefield victories have helped the ruling party win all local elections despite economic hardships attributed to the war effort in which tens of thousands of people on both sides have died. 

"The main reason for the success is from day one, we maintained a clear mission," the defence secretary said. "We maintained it without ambiguity: that is to finish off the LTTE."