A crowd of civilians in Sri Lanka were the target of a woman suicide bomber on Monday.

The military said the woman had a bomb strapped to her body and hid in the crowds before blowing herself up in the northern war zone killing 28 people, the Associated Press reported.

The suicide attack, which killed 20 soldiers and eight civilians, led to fears the Tamil Tiger separatists — boxed into a small strip of land on the northeastern coast and on the verge of defeat — will increasingly turn to guerrilla warfare in their battle against government forces.

According to Red Cross estimates, 250,000 civilians are believed to be trapped in the area.

The rebels are accused by the military of using civilians as human shields. The military has 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 AP reported.

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 attack has been condemned by the United Nations.