The attack, which also wounded 50 people, came after a weekend of heavy shelling that killed hundreds of civilians trapped in the tiny war zone. The military denied shelling the coastal strip under rebel control, which is packed with an estimated 50,000 civilians.
Dr Thurairaja Varatharajah, the top government health official in the war zone, said a single mortar shell hit the admissions ward in the makeshift hospital Tuesday morning. The death toll was expected to rise, he said.
Shells were still hitting the area hours later, including one that landed about 150 yards (meters) from the hospital, Varatharajah said.
Just outside the admissions ward - little more than a corrugated tin roof with blue tarp walls - bloody bodies were strewn about in the dirt while health workers hooked up the wounded to IV lines, according to photographs taken after the attack. Later, nearly two dozen dead bodies were lined up in rows in a sandy courtyard.
Other photographs showed civilians fleeing the area. One man running away was carrying a child with a bandaged head.
Two other hospital officials, who spoke separately on condition of anonymity because they were not authorized to speak to the media, confirmed the attack and said a hospital administrator was among those killed.
It was the second time this month the facility has come under heavy fire. On May 2, 64 civilians died when the hospital was hit by artillery.
Meanwhile, army troops broke through a sand fortification the Tamil Tigers had built in the area, killing dozens of insurgents and advancing further into what little remains of rebel-held territory, military spokesman Brig. Udaya Nanayakkara said.
Reports of the fighting are difficult to verify because the government bars journalists and aid workers from the war zone.
Rebel spokesman Seevaratnam Puleedevan blamed the attack on hospital on the government, and said civilians were fleeing in all directions inside the tiny war zone, seeking safety.
"There´s no place to seek shelter or protect themselves," he said.
Sri Lankan defense spokesman Keheliya Rambukwella denied the army had launched the attack and reiterated the government´s promise not to launch any airstrikes or artillery into the densely populated area.
The shelling came as a Red Cross boat sent to deliver food aid and evacuate the wounded waited off shore, the health officials said.
"There is fighting going on and we need a more quiet environment to land," said Paul Castella, the head of the Sri Lanka office for the International Committee of the Red Cross.
Castella said if the fighting did not soon subside, the boat would be forced to turn back.
The rebels called on the international community to force the government to stop its offensive against the violent separatist group, which has been fighting for a homeland for the ethnic Tamil minority for more than a quarter century.
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