Up to 13 Sri Lankan sailors are presumed dead after a navy boat was ambushed by suspected Tamil Tiger rebels, officials say.
Tamil Tiger sailors on patrol
A fishing boat crammed with explosives rammed one of two patrol vessels as they left the port of Trincomalee.
Earlier reports said 15 sailors had died but a navy spokesman later said two had been found alive.
The attack comes amid growing concerns that Sri Lanka's four-year old ceasefire will soon break down.
"The incident has brought the ceasefire under pressure like never before," Hagrup Haukland, the head of the Scandinavian monitoring mission told the Associated Press news agency.
"The question is how long the Sri Lankan security forces can take it."
The BBC's Dumeetha Luthra, in Colombo, says that the rebels are believed to be behind the suicide attack, on the country's north-eastern coast, although they have not claimed responsibility.
"The boat exploded and we believe it is an LTTE (Liberation Tigers of Tamil Eelam) suicide attack," defence ministry spokesman Prasad Samarasinghe said, the AFP news agency reports.
"There were two gunboats in the area and one saw the other being attacked."
Recent attacks blamed on the Tamil Tiger rebels have raised fears that Sri Lanka may be sliding back into civil war after four years of an official ceasefire.
December was the bloodiest month in those four years, with 39 troops killed by the Tigers.
The peace process has been hamstrung since Sri Lanka's presidential election in November.
The government of new President Mahinda Rajapakse has been unable to agree on a venue for peace talks with the rebels.
More than 100 people have been killed in the past month
On Friday around 10,000 people in Trincomalee attended the funeral of five Tamil students who were killed there earlier this week.
A post-mortem examination showed that the students had gunshot wounds, and the rebels have accused the Sri Lankan security forces of carrying out the killings. The army has denied any involvement.
The Tamil Tigers want a separate state in the north and east of Sri Lanka.
More than 60,000 people have died during two decades of conflict with Sri Lanka's government.