At least 10 sailors and their driver have been killed after a navy bus hit a mine in north-eastern Sri Lanka.
Another nine sailors were hurt in the blast near Trincomalee, six of them critically. Two British women in a passing vehicle were also hurt.
Military officials are blaming Tamil Tiger rebels for the blast, a day after five soldiers and two civilians died in a mine attack near Jaffna.
The rebels have denied involvement in both attacks.
Talks 'in jeopardy'
Military spokesman Brigadier Prasad Samarasinghe told the BBC that the bus hit a claymore mine as it was taking the sailors on holiday.
"Two attacks within days - it is very worrying"
Earlier reports spoke of 12 people killed, but Brig Samarasinghe said one of those was among the critically injured.
The Foreign Office said the two British women, a mother and daughter, were travelling with another Briton and a Sri Lankan when the bus veered into their vehicle.
Neither women suffered life-threatening injuries.
The women were transferred to Colombo in a Sri Lankan air force helicopter, the Associated Press news agency reported.
A rebel spokesman condemned the incident. "We have nothing to do with the blast," he told the BBC.
Ceasefire monitors said they were investigating the attacks.
"At this stage it is safe to say that these attacks are seriously jeopardising not only the Geneva talks scheduled later this month but are having a detrimental effect on the ceasefire," they said in a statement.
Fragile ceasefire
The government and the rebels are due to meet in Geneva later this month for a second round of peace talks aimed at shoring up their threadbare truce.
A surge in attacks at the turn of the year abated after agreement was secured to hold talks in February, but violence has risen again in recent weeks.
More than 120 people - including about 80 soldiers and sailors, and many civilians - were killed in December and January.
Tamil Tiger supporters said more than 40 Tamils were killed by the security forces during that period. Others blamed some of those deaths on the rebels or other armed groups.
The rebels have denied targeting the armed forces, describing attacks on soldiers and sailors as a "popular uprising" of the Tamil people. BBC correspondents say few believe that such attacks can take place without the Tigers' knowledge.
The Tamil Tigers want autonomy for minority Tamils in the north and east of Sri Lanka. More than 60,000 people have died during two decades of conflict.
On Monday, the Tamil Tigers were listed as a terrorist group by Canada, home to the largest expatriate community of Sri Lankan Tamils in the world.
Observers say the decision is a major setback for rebel fund-raising efforts. The UN, US and UK have already proscribed the Tigers.
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