At least 61 people have been killed after a civilian bus was struck by a mine in northern Sri Lanka, police say.
Another 45 people were wounded in the explosion in Anuradhapura district, 200km (125 miles) north of Colombo.
T
he government says the Tamil Tigers were responsible for what is the worst incident involving civilians since a 2002 ceasefire came into force.
The Tigers deny responsibility. The government has responded with air strikes on Tamil Tiger positions.
The Tigers say the attack may be the work of a paramilitary group linked to the government.
Violence has risen in Sri Lanka since early April when talks between the rebels and the government broke down.
"We have to seriously consider the ceasefire agreement and possibly restructure it," government spokesman Keheliya Rambukwella said.
Children dead
Thursday's bus attack was in an area with an ethnic Sinhalese majority. Correspondents say the Tamil Tigers have in the past attacked the main military barracks in the area.
The bus was packed with villagers traveling to work and school when it was hit by a claymore mine, officials said.
Army officials said the bus overturned and hurtled 25 meters down a road after it hit the mine in the town of Kabithigollewa in Anuradhapura district.
A doctor at the local hospital said at least 15 children were among the dead. A cameraman for Reuters news agency says he also saw the corpses of many women.
"This is the most barbaric attack of the Tigers," Mr Rambukwella said.
The Tigers immediately issued a statement denying responsibility saying that "directly targeting civilians... cannot be justified under any circumstances".
Claymore mines are widely used by the Tamil Tigers. But a Tamil Tiger (LTTE) spokesman, S Puleedevan, told the BBC's World Update programme that the attack may have been the work of a paramilitary group linked to the government.
"The Sri Lankan armed forces are using various paramilitary groups. They are engaged in a lot of claymore attacks, penetrative attacks, against the LTTE, against the Tamil civilians, in the north-east," Mr Puleedevan said.
Anuradhapura was the ancient capital of Sri Lanka. Correspondents say Thursday's attack has extra significance as it was carried out in the Sinhalese Buddhist heartland.
Talks failure
Shortly after news of the attack emerged, the Sri Lankan military launched air strikes in rebel-held areas of Sampur and Kilinochchi.
The pro-Tiger TamilNet website said two air force bombers had begun bombing rebel-controlled Mullaitivu and its suburbs.
It is not clear what impact the air attacks have had.
Suspected attacks by Tamil Tigers on security forces and killings of Tamils blamed on the army and others have soared in recent months.
Earlier this month talks the rebels refused to meet the Sri Lankan government side in talks in Oslo mediated by Norway. The Tiger team only returned to Sri Lanka on Wednesday.
At the end of May the European Union added the Tigers to its list of banned terrorist organizations.
The Tamil Tigers want a separate homeland for minority Tamils in the north and east of Sri Lanka. More than 60,000 people have died during two decades of conflict.
Photo Caption
The wreckage of Thursday's bus attack