COLOMBO, Sri Lanka - Video footage captured from Sri Lanka's Tamil Tiger rebels showed their fighters forcing civilians to assist their war effort and depicted one insurgent in street clothes firing a heavy machine-gun, the military said Thursday.

The government said the footage, posted on the Defence Ministry website, proves that the rebels have been putting civilians in the line of fire and have shed their military uniforms so they would be mistaken for non-combatants themselves if killed.

A United Nations report last month said nearly 6,500 civilians had been killed in three months of fighting this year, and health officials in the war zone say many more have been killed since then. Diplomatic pressure has built on the government to temporarily halt its offensive to allow an estimated 50,000 civilians trapped in the war zone to flee.

President Mahinda Rajapaksa, speaking to foreign diplomats Thursday, reiterated his opposition to a ceasefire.

The government says it is on the verge of defeating the rebels and such a pause would allow them to regroup. It has denied the reports of heavy civilian casualties and accused the rebels of holding the civilians as human shields.

The video showed a man, apparently a Tamil Tiger fighter, wearing a white, buttoned shirt, firing a heavy machine-gun mounted on an armoured vehicle as others in street clothes helped him manoeuvre.

Narration provided by the government said the advancing troops captured the vehicle, killing all the combatants near the village of Mulliavaikkal and recovering the videotape that included footage of the body of a slain rebel.

The video also showed men in civilian clothing camouflaging the armoured vehicle with tree branches and a line of people carrying hoes and walking behind a rebel fighter. The narrator said the civilians were being taken to dig trenches and erect earth fortifications against the advancing government forces.

Defence spokesman Keheliya Rambukwella said the footage backed up the government assertions that the rebels were trying to discredit the government by making it appear as if many innocent civilians were being killed.

"This has been going on for a long time," he said.

The rebels could not be reached for immediate comment.

In recent months, government forces have ousted the rebels from their strongholds across the north and cornered them in a tiny sliver of land along the northeast coast.

UN Secretary General Ban Ki-moon became the latest international leader to ask President Mahinda Rajapaksa to suspend the offensive to allow aid into the war zone. Rajapaksa has brushed off calls for even a brief ceasefire.

In the phone call between the two leaders late Tuesday, Rajapaksa invited Ban to come to Sri Lanka and inspect the situation for himself.

UN spokeswoman Michele Montas told reporters in New York that no decision has been made on such a visit, but "the secretary general feels that if it can save lives, he will definitely consider it."

Rajapaksa told diplomats that a ceasefire with the Tamil Tigers would be "useless" and vowed to fight on and destroy the group.

He said the rebels have used previous ceasefire periods to rearm and regroup and said the government "will not be deceived anymore."

"The LTTE only believe in the language of terror, which needs to be met by military force," he said.

Fighting, meanwhile, continued Thursday, with soldiers capturing an earthen fortification in the shrinking rebel enclave, the military said. Troops also found the bodies of three slain Tamil Tiger fighters, the military said.

It is not possible to independently verify the military's claims because reporters and independent observers are barred from the war zone.

The rebels have been fighting since 1983 for a separate state for minority Tamils, who have suffered decades of marginalization at the hands of governments controlled by the Sinhalese majority.