"At least 20 people have been killed and several are injured.
It was a state-run bus with at least 60 civilians on it, including schoolchildren," Sri Lankan military spokesman Udaya Nanayakkara said.
The defence ministry said the bus, travelling from Buttala to Okkampitiya, was hit by a fragmentation bomb apparently hidden on the bus.
In all 50 people were killed or injured, it said.
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It blamed the attack on the Liberation Tigers of Tamil Eelam (LTTE).
"LTTE terrorists have carried out a cowardly bomb blast targeting innocent schoolchildren at Okkampitiya, Monaragala," the ministry said, describing the rebels as "a ruthless terrorist outfit notorious for crimes against women and children."
The attack came as a 2002 Norwegian-brokered truce, which was in practice dead anyway, is set to officially end on Wednesday.
The government abandoned the ceasefire two weeks ago, saying there was no point attempting to negotiate with "terrorists.
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Sri Lankan government defence offi