Al Jazeera's David Hawkins, reporting from Sri Lanka, said the bomber detonated her explosives as she was being frisked by soldiers.
The government had previously been criticised for putting civilians who left the war zone into military camps, he said.
"[Government officials] say they have to check the civilians very carefully to make sure Tamil Tiger fighters are not trying to sneak out along them," he added.
"This suicide bomb attack shows that those concerns are legitimate."
The United Nations and aid agencies have expressed concern for the estimated 250,000 civilians trapped in the sliver of land still controlled by the Tigers.
|Both sides deny targeting civilians in the ongoing conflict [AFP]
The government has denied targeting civilians, claiming only 120,000 people are trapped, and has said food and water has been provided at the frontlines for those fleeing the conflict.
International organisations including the Red Cross have urged both sides to let noncombatants out of the conflict zone.
The government, aid agencies and rights groups have accused the rebels of forcibly keeping people in the war zone as human shields, conscripts and labourers, which the Tigers deny.
The pro-rebel website TamilNet alleged the military had shelled groups of civilians, killing more than 120 people on Friday and Saturday.
It is impossible to independently verify the two sides' claims as journalists are banned from the war zone.
The Tamils have waged a 25-year guerrilla campaign against the majority Sinhales who dominate Sri Lankan politics.