At least 15 Sri Lankan navy sailors were missing after their patrol boat was sunk Thursday by Tamil Tiger rebels off the northern coast, the navy said. In retaliation, the navy sank five rebel vessels, it said, in a sharp escalation of violence between the two sides.

The air force also bombed areas near the rebels' northern headquarters, a rebel spokesman said.

The patrol boat was part of a convoy escorting a troop carrier that was attacked by the Liberation Tigers of Tamil Eelam rebels, navy spokesman Commander D.K.P. 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. "There were 15 to 20 sailors in the boat."

The military sank five rebel vessels in retaliation, it said.

CountryWatch: Sri Lanka

Helen Olafsdottir, spokeswoman for the European-led Sri Lanka Monitoring Mission, said a truce monitor was in the vessel with the soldiers. Other details were not immediately available, she said.

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 LTTE has made what SLMM feels are 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.

In a statement, the Defense Ministry said Tamil Tiger rebels threw grenades at an army checkpoint in Jaffna, wounding one soldier and triggering a gunfight in which one of the attackers was also injured.

Also in Jaffna, about 190 miles north of the capital, Colombo, suspected separatist rebels attacked a military bunker with grenades and small arms fire, injuring a soldier and killing a Tamil civilian, the ministry said.

In Muttur, close to Trincomalee, unidentified gunmen fatally shot Kandasamy Jayasurendran, 21, a Tamil civilian, said area police officer D.M. Abeywardena.

Police said they were unaware of the motive for the killing but friends of the victim, who spoke on condition of anonymity for fear of retribution, said Jayasurendran was close to Tamil Tiger rebels.

On Tuesday, 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.