Sri Lanka's air force bombed Tamil Tiger rebel positions for a second day Friday, as ground troops exchanged mortar fire with insurgents in a battle over water supplies for thousands living in government-held villages, officials said.

There were no immediate reports of casualties from Friday's air raids. Similar attacks on Thursday killed six insurgents and wounded five, according to the rebels.

In the latest violence to threaten the island nation's increasingly fragile 2002 cease-fire, the Tigers fired mortar rounds at government troops who were advancing toward a rebel-held area where the insurgents had blocked water from a reservoir -- the main source of water for 15,000 families living in government-controlled villages.

The rebels justified their action by saying the government had reneged on a promise to build a water tower for areas under rebel control.

There were contradicting versions of Friday's violence.

"We have stopped the advance of Sri Lankan forces and we are in complete control," rebel spokesman Rasiah Ilanthirayan said of the rebels' hold over a sluice gate that regulates water from a reservoir in the Trincomalee district.

The government disputed the claim that the military's advance had been halted, and spokesman Keheliya Rambukwella said troops were following a plan.

"There is a plan designed by the military to achieve the target, which is to supply water to the civilians,' Rambukwella said.

He said air force jets pounded rebel positions in the area to weaken the insurgents. He had no details.

Earlier, security forces blocked a group of five Buddhist monks and nearly 400 villagers who were trying to reach the water plant in Trincomalee on Friday, because of concerns over their safety.

The monks want to reopen the sluice gate so that water can flow to the villages, said Udaya Gammanpila, a spokesman for Jathika Hela Urumaya, a nationalist group that opposes the rebels.

Most of Sri Lanka's 14 million Sinhalese are Buddhist while the country's 3.2 million ethnic minority Tamils are mainly Hindus.

Earlier, the pro-rebel Web site TamilNet said six rebels were killed and five others wounded Thursday when government airstrikes hit a camp in Kathiraveli, 40 kilometers (25 miles) southeast of Trincomalee town.

The military also conducted a separate air raid on what it described as an unfinished rebel runway. The government said it acted on information that the insurgents had cleared forests and were building an unpaved air strip in the northern rebel-held town of Mullaittivu.

Late Friday, suspected rebels exploded a roadside bomb, killing three village guards in Anuradhapura district, 160 kilometers (100 miles) northeast of Colombo, bordering the country's restive north and east, the military said on its Web site.

Separately Friday, two Sri Lankan policemen were shot to death in the northeastern town of Trincomalee. The military blamed the rebels for the attack.

Elsewhere, a scooter rickshaw packed with explosives blew up in a rebel-held area of eastern Batticaloa district, said D. Ranasinghe, a senior police officer in the area. Rebel spokesman Ilanthirayan said two unidentified people inside the rickshaw died in the explosion.

The rebels started fighting for an independent Tamil homeland in 1983, accusing the Sinhalese of discrimination. A cease-fire in 2002 halted the conflict, but subsequent peace talks broke down over rebel demands for sweeping autonomy.

An escalation of violence in recent months has threatened a return to all-out war with more than 750 people -- half of them civilians -- killed since December.