Mounting attacks have raised fears of a return to all-out civil war on this tropical island off the southern tip of India.

Late Friday, the Tamil Tiger rebels' sea wing appeared to be about to attack a strategic port in the northern Jaffa Peninsula, a navy spokesman said.

"We first made some warning shots, but the boat continued and then we fired and the boat caught fire and sank," Commander D.K.P. Dassanayake said.

It was not immediately known how many people were on the boat, or if there were any rebel casualties.

"Our men had to fire because the boat was advancing toward Kankesanturai harbour."

No non-navy boats are allowed in the waters because it is designated a "high security zone," Dassanayake said. The harbour is a main point of entry for supplies to an estimated 40,000 troops stationed in the region.

A pro-rebel Web site, TamilNet, called the incident a "sea fight," and reported that two boats were seen burning, although it did not identify the vessels.

Dassanayake said no navy vessels were damaged.

On Saturday, the army killed a suspected Tamil guerrilla as he tried to attack a checkpoint with a hand grenade in northern Mannar district, 135 miles north of the capital, Colombo, said Brig. Prasad Samarasinghe, an army spokesman.

The army suffered no casualties, he told The Associated Press.

Separately, suspected Tamil Tigers using small arms and mortars attacked an army road clearing patrol in Muttur, a small town in eastern Sri Lanka, Samarasinghe said.

"We also retaliated using mortars," he said, adding that no soldiers were wounded in the clash near the port city of Trincomalee, about 135 miles northeast of Colombo.

The rebels did not immediately comment on Saturday's attacks.

More than 700 people have been killed in violence since April, despite a 2002 cease-fire agreement between the government and the rebel group.

The Tigers have been fighting since 1983 to carve out a separate homeland for Sri Lanka's 3.2 million minority Tamils.