The civilians were killed when shells hit three schools where they were sheltering after security forces asked them to move to public buildings while troops mopped up Tamil Tiger rebel resistance, officials said.
Military spokesman Upali Rajapakse said the bloodiest attack was against Arabic College in Muttur where 10 people were killed and at least 50 were wounded.
He said two other Muslim schools at Thoppur, a suburb of Muttur, were also hit by shells leaving five dead and 10 wounded at Al Noori College and two killed at Al Hameema College.
"The Tigers are trying to bring down artillery on our positions and when they miss, this is what happens," Rajapakse said.
Two constables and two paramilitary troopers attached to the local police were also killed in the clashes in Muttur, officials said.
The defence ministry said the shell that killed the 10 civilians came from the Tigers while the pro-rebel Tamilnet.
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com website said the attack had been initiated by the military.
The Tamiln