Sri Lanka's government says Tamil rebels have attacked a troop transport ship in the northeast, while the air force launched a new offensive to free a rebel-held waterway.

Reports and a pro-rebel web site, Tamilnet.com, say at least four sailors were killed when the ship carrying more than 800 troops came under heavy mortar fire Tuesday.

The Sri Lankan military said the ship was on the way from northern Jaffna peninsula to the port of Trincomalee when it came under attack, but that it had managed to sail to safety.

Villagers look at paddy fields that dried up after Tamil Tiger rebels cut off water, in Trincomalee, Tuesday

Meanwhile, Sri Lankan warplanes continued pounding Tamil positions in the same district in an attempt to free up a rebel-held irrigation canal.

The military says the fighting will go on until the flow of water resumes for 60,000 farmers living on government-held land. That confrontation has left at least seven soldiers and 35 rebels dead.

Also Tuesday, Sweden decided to pull out its cease-fire monitors from the island nation, joining Finland and Denmark which made similar announcements last week.

On Monday, a roadside bomb that blew up an army bus in the same area Monday had killed 15 soldiers and two civilians.

Also Monday, the leader of the Tamil Tigers' political wing in Trincomalee said the four-year cease-fire with Sri Lanka's government was dead and blamed army assaults for forcing a return to civil war.

Some information for this report was provided by AFP, AP and Reuters.