Sri Lankan government forces have shelled a hospital inside the conflict zone killing at least 45 people, Tamil Tiger rebel and hospital sources say, BBC reported.
The rebels said the makeshift hospital in Mullivaikal was hit early in the morning, and people injured in weekend attacks were among the dead.
The government has repeatedly denied shelling the war zone, saying it is rebels who are attacking civilians.
The claims are impossible to verify as reporters are banned from the area.
More than 400 people were killed and over 1,000 injured over the weekend in what the UN has described as a bloodbath.
Many of the casualties from that fighting were taken to the Mullivaikal hospital.
Thurairaja Varatharajah, a health official at the hospital, told the Associated Press news agency it had been hit by a single mortar shell.
More than 50 people were injured, and he expected the death toll to rise because many of the injured had bad head and stomach wounds, he added.
However, military sources have suggested that doctors in the zone may be being forced to give statements at gunpoint by rebels.
Rebel spokesman Seevaratnam Puleedevan told AP civilians were fleeing in all directions.
"There's no place to seek shelter or protect themselves," he said.
The Sri Lankan military has meanwhile blamed civilian deaths on the Tigers, saying they are using people as human shields.
The UN estimates that about 50,000 civilians are trapped by the conflict, in a three-sq-km strip of land.
UN Secretary General Ban Ki-moon has called for an immediate halt to the fighting.
The Tamil Tigers have fought for an independent homeland for Sri Lanka's Tamil minority since 1983.
More than 70,000 people have been killed in the war.