COLOMBO, Sri Lanka - A suspected Tamil Tiger rebel who pretended to be a war refugee blew herself up Monday as Sri Lankan soldiers frisked her at a checkpoint. Twenty troops and eight civilians died.

State TV showed the carnage after the suicide bombing in Vishwamadu, a northeastern town where hundreds of civilians had been waiting to be sent to refugee camps: a woman in a blue dress curled up in the fetal position, her face and neck spattered with blood; plastic lawn chairs upended and piled in a jumble from the force of the blast.

A soldier briskly picked up a dead child who was sprawled face down in the dirt, yellow shorts peeping out from beneath her bloodstained pink-and-purple dress. He dropped her rag-doll body on top of another corpse in a truck, leaving their bloodied, bare feet jutting out the back.

The footage, released by the government, did not show the bodies of any soldiers.

The military has accused the rebels of using the civilians as human shields and called for noncombatants to flee to government-controlled areas. The rebels have accused the government of indiscriminate shelling, including in a government-designated "safe zone," leading to increasing civilian casualties.

On Monday morning, more than 800 civilians had crossed the front-lines and were being searched by soldiers when the bomber attacked, military spokesman Brig. Udaya Nanayakkara said.

The accused bomber was being frisked when she set off the explosives, killing 28 people and wounding 24 troops and 40 civilians, he said.

The attack targeted a military weak point: the processing of the masses of civilians fleeing the area.

Military officials say the flow of civilians out of the war zone has increased in recent days, with 4,700 fleeing Sunday, bringing the total number of noncombatants to escape the war zone to 20,000 this year, Nanayakkara said.

The attack also highlighted concerns that the rebels were trying to blend in with the civilian population so they can fight on using insurgent tactics.

"The LTTE is now desperate because they don't have any control over the civilians," Nanayakkara said, calling the rebels by the acronym of their formal title, the Liberation Tigers of Tamil Eelam. "They wanted to stop these people coming in."

With most communication to the north severed, the rebels could not be reached for comment.

The U.S. Embassy in Colombo called on the rebels "to allow all civilians freedom of movement" and urged the Sri Lankan government to ensure that all civilians who flee the fighting are transferred to the camps "in accordance with international standards."

The United Nations also condemned the bombing.

"We deplore the loss of civilian life in this targeted killing. It's a blow for people who have suffered so much," said UN resident co-ordinator Neil Buhne.

The rebels have been accused of more than 200 suicide attacks in 25 years of civil war.

Hundreds of thousands of Sri Lankans, meanwhile, crammed into a patriotic exhibition displaying weapons, boats and even submarines captured from the rebels, underscoring growing optimism that decades of war could be drawing to a close.

"We are certain the end of the LTTE is very near. Look at the weapons captured from the Tigers," said Sumith Samarasinghe, 37, a small business owner.

The Tamil Tigers began fighting in 1983 for a separate state for the country's ethnic Tamil minority after decades of marginalization at the hands of governments controlled by the Sinhalese majority.

More than 70,000 people have been killed in the fighting.