"Yesterday's massive attack on civilians shows that force continues to be used indiscriminately in the conflict in Sri Lanka," said Egeland, the UN under secretary general for humanitarian affairs.
"Equally disturbing are reports that the rebel Liberation Tigers of Tamil Eelam (LTTE) prevented some 2,000 civilians from fleeing to safety," Egeland said.
"I call upon all parties to the conflict to ensure the protection of civilians under all circumstances in accordance with International Humanitarian Law," he added.
Wednesday's Sri Lankan army artillery bombardment hit a school that was sheltering some 1,000 internally displaced persons (IDPs) in the Vakarai area in Batticaloa District of eastern Sri Lanka.
Sri Lankan President Mahinda Rajapakse ordered a probe into the artillery attacks and expressed "his deep sorrow and regret at the senseless loss of life of innocent civilians at Vakarai."
The president condemned "the LTTE's tactics of directing long range artillery fire using human shield that led to this tragedy," the government said in a statement.
The fighting between the LTTE and the Sri Lankan government has displaced almost 200,000 people since April 2006. In addition, hundreds of thousands of Sri Lankans are displaced due to previous conflicts and the 2004 tsunami.

Copyright AFP (Agence France-Presse), 2006