Sri Lanka's navy says it has destroyed two Tamil Tiger boats off the north-eastern coast, killing 15 rebels.
The navy said the clashes happened near Trincomalee on Tuesday night. Rebels say they are investigating the reports.
In a separate incident to the south, the navy attacked a large ship believed to be carrying arms to the rebels.
Violence has soared in Sri Lanka in recent months as the country has slid back towards war, despite a 2002 ceasefire officially still in place.
On Tuesday, the US and Italian ambassadors were hurt in a mortar attack that rebels say was a mistake.
More than 4,000 people have been killed since late 2005. The rebels have said they are compelled to renew their struggle for independence.
The sinking of the boats came days after the fifth anniversary of the ceasefire, which has unravelled over the past year.
Defence spokesman Brigadier Prasad Samarasinghe told the BBC News website that 14 rebel boats coming from Mullaitivu had been detected trying to enter the port at Trincomalee.
Two rebel boats were destroyed and another four damaged in the fighting that followed, he said. Two navy personnel were injured.
In the other sea clash, the navy said it detected what it called a suspicious ship in the deep sea around 180 nautical miles south of the island on Wednesday morning.
After a challenge and warning shots, the navy patrol fired on the 70-metre-long vessel, setting it ablaze.
Navy spokesman DKP Dassanayake said there were several explosions on board as it burned - and he believed the ship was carrying artillery shells for the rebels.
Rasiah Ilanthirayan, military spokesman for the Tigers, said he was investigating the navy reports.
In another incident, Sri Lanka's air force attacked a rebel base in an eastern jungle, Brig Samarasinghe said. There was no information on whether there were any casualties.
"The rebels were bringing in mortars and arms to this base," he said.
The Tigers want an independent homeland for Tamils
On Tuesday US ambassador to Sri Lanka Robert Blake and his Italian counterpart Pio Mariani were slightly injured by rebel mortar fire as they got off a helicopter at an air base in Batticaloa, where they were to attend a development meeting.
The Tigers said they were responding to a government artillery attack and were not informed in advance that foreign dignitaries were visiting conflict areas.
Shelling between the two sides continued on Wednesday.
In recent months government forces have driven the Tigers from large areas in the east of the country.
The rebels have withdrawn to jungle areas.
At least 64,000 people have died since the rebels began their fight for independence more than 20 years ago.