At least 50 rebels were killed and 17 Sri Lankan sailors missing after a sea battle Thursday instigated by the Tamil Tigers left the country on the brink of civil war.

Tamil Tigers sank a navy patrol boat off the northern coast as it escorted a troop transport carrying 710 soldiers. In retaliation, the navy downed five rebel vessels and the air force launched airstrikes on guerrilla-held territory.

CountryWatch: Sri Lanka

The escalation in violence could mark a return to civil war, as a 2002 cease-fire that stopped almost two decades of fighting appears increasingly unlikely to last.

"This is a very serious attack (by the Tigers), a blatant violation of the cease-fire agreement," government spokesman Keheliya Rambukwella told The Associated Press.

The patrol boat was part of a convoy escorting a troop carrier that was attacked by the Liberation Tigers of Tamil Eelam rebels, Dassanayake said.

"About 15 LTTE boats including suicide boats attacked one of our vessels transporting 710 soldiers," Dassanayake said.

"Navy fast-attack boats escorting the vessel engaged the Tiger boats and one of them was destroyed by a suicide boat," he said, adding there were about 20 sailors on board the stricken vessel.

A search was ongoing late Thursday for the missing sailors who "made worthy efforts to save hundreds of soldiers who were on board the main vessel," Dassanayake said.

At least 50 Tiger guerrillas were on the sunken rebel boats and all were believed dead, Dassanayake said. A pro-rebel Web site quoted unnamed rebel sources as saying they lost only four guerrillas in the battle.

No independent verification was immediately possible.

Rebel spokesman Daya Master said air force fighter planes "attacked two times, dropping bombs in our territory." The bombs fell a few miles from Kilinochchi, he said by telephone from the rebels' stronghold in the town, 170 miles north of Colombo.

Helen Olafsdottir, spokeswoman for the European-led Sri Lanka Monitoring Mission, said a truce monitor was in the troop carrier with the soldiers.

"This is very serious," she told BBC television. "It is getting worse."

Dassanayake said the troop carrier and the truce monitor on board were unharmed.

In a statement, the monitors accused the rebels of violating their 2002 cease-fire agreement with the government and said they considered the attack a direct threat to their mission.

The Tiger rebels made "threats to our monitors, warning them not to participate in patrols in navy vessels," the monitors said.

"This sort of reckless behavior can only lead to a dangerous escalation resulting in growing hostilities and jeopardizing any possibility for future peace talks," they said.

Also Thursday, two civilians died and at least three other people were wounded in three attacks blamed on the separatist rebels in the north and northeast.

The killings took place two days after Japanese peace envoy Yasushi Akashi failed to convince the rebels to resume peace talks with the government.

Akashi, Japan's special envoy to Sri Lanka since 2002, said relations between the government and the rebels have plunged to the worst level since their 2002 cease-fire.

More than 150 people have died in violence since the beginning of April.

The Tigers began fighting in 1983 to create a separate state for ethnic minority Tamils, accusing the majority Sinhalese of discrimination. More than 65,000 people died in the conflict before the 2002 truce.