At least 16 patients were killed in the shelling of a makeshift hospital in the war zone in northeastern Sri Lanka, Red Cross officials said.
The area near the village of Putumattalan was hit by shelling Monday, the Red Cross said in a statement Tuesday.
Red Cross officials called on both the government and the rebels to refrain from attacks on the wounded and medical personnel.
The Red Cross evacuated 240 patients from the area by sea and hoped to evacuate the remaining 160.
The United Nations, meanwhile, said it was outraged by the "unnecessary" deaths of hundreds of people inside rebel territory and urged both sides to avoid fighting in civilian areas.
The government accuses the Tamil Tiger rebels of holding civilians in the war zone hostage to use as human shields against the military's offensive. The rebels deny the accusation.
International human rights groups say more than 200,000 civilians are believed trapped in the patch of territory still under rebel control.
Military spokesman Brig. Udaya Nanayakkara said the insurgents fired early Tuesday on a group of more than 1,000 trying to escape the war zone in Udayarkattu, inside what remains of rebel-held territory. The attack killed 19 and wounded another 75, the military said.
Confirmation of the reported shooting was not possible because independent journalists and nearly all aid workers are barred from the war zone. The rebels could not be reached for comment because communications to the north have largely been severed.
Patients under siege
Meanwhile, local fishermen helped Red Cross workers ferry about 240 patients to a specially chartered boat anchored offshore from the village of Putumattalan in the war zone, said Sarasi Wijesinghe, a spokeswoman for the International Committee of the Red Cross.
The patients' injuries were making the operation difficult, she said.
"Most of them can't sit upright. They have to be lying down. A lot of care has to be exercised," she said.
The aid group chartered the ferry, which was flying the Red Cross flag, from the government-controlled town of Jaffna in the north, and was in contact with both sides to ensure no fighting interfered with the evacuation, Wijesinghe said.
The boat was headed south to a port in Trincomalee, she said.
The patients had fled the last functioning hospital in the war zone in Puthukkudiyiruppu last week after it came under repeated artillery barrages that killed several patients.
The intense fighting between the government and the Tamil Tiger rebels made fleeing south across the front lines too risky, so the group instead headed deeper into rebel-held territory.
War zone violence
The Red Cross and government doctors set up a makeshift medical facility in an abandoned community center and a school, Wijesinghe said. However, the area was shelled Monday, she said.
In addition, the overcrowded center does not have a reliable supply of drinking water, and "the lack of sanitation and hygiene is a problem," she said. "Some patients are lying on the ground, the floor."
In recent days, the military has reported an increasing flow of civilians out of the war zone.
More than 1,000 civilians fled Tuesday, and 6,599 reportedly crossed Monday, even as a female bomber killed 19 soldiers and 10 civilians at an army checkpoint. The government earlier said the blast had killed 20 soldiers and eight civilians.
Amnesty International condemned the attack as a clear violation of international law.
"Blurring the distinction between civilians and combatants means that thousands of ordinary people, desperate to flee the conflict area, are at greater risk of reprisals and getting caught in crossfire," said Yolanda Foster, the London-based group's Sri Lankan researcher.
Rights groups have also accused the government of killing and wounding civilians by firing artillery into the increasingly cramped war zone in a small pocket of the northeast.
On the verge of ending war?
The Tamil Tiger rebels have been fighting since 1983 for an independent state for minority Tamils. Government troops have forced the rebels into a broad retreat in recent months and officials say they are on the verge of crushing the insurgency and ending a war that has killed more than 70,000 people.
In Geneva, the U.N. refugee agency said it was "outraged by the unnecessary loss of hundreds of lives and the continued suffering of innocent civilians" inside rebel territory. It urged the government and Tamil Tigers to avoid fighting inside a government-designated "safe zone" where many civilians were reportedly killed.
"Without respect for international humanitarian law by both parties, the bloodshed will continue," the U.N. High Commissioner for Refugees said.