A senior military official says Sri Lankan troops are fighting heavy battles with the rebels over the key Elephant Pass base on the edge of the Jaffna peninsula.

The official said that troops surrounded the base and entered it from the north and south on Friday. They expected to take complete control of the area soon. The official spoke on condition of anonymity because he was not authorized to speak to the media.

The capture of Elephant Pass would be a major victory for government forces a week after they seized the rebels' administrative capital of Kilinochchi.

The rebels were not available for comment.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

COLOMBO, Sri Lanka (AP) — Sri Lankan troops battled the Tamil Tigers in their last remaining strongholds in the north, while the rebels detonated a roadside bomb Friday in the east that killed seven people.

The rebels triggered the bomb as a convoy escorted by soldiers passed by, said military spokesman Brig. Udaya Nanayakkara. The blast killed three air force troops and four civilians and wounded six others, he said.

Rebel officials could not be reached for comment.

The assault outside the eastern city of Trincomalee signaled the rebels were turning to guerrilla tactics. The government captured the east from the rebels in 2007, but attacks in the area have increased in recent months.

Meanwhile, the soldiers who captured the rebels' administrative capital of Kilinochchi last week pushed eastward from the town toward new rebel defense lines, Nanayakkara said.

Military forces were also closing in on the strategic rebel base at Elephant Pass, the gateway to the northern Jaffna peninsula. Most of Jaffna, the cultural capital of the country's ethnic minority Tamils, is in government hands, though the rebels retain a shrinking stronghold on the peninsula's southern edge.

On Thursday, troops sweeping in from the north overran the rebel base at Pallai on the peninsula and then marched on, capturing Sorampattu, several miles (kilometers) to the south, the military said.

The government has said it hopes to crush the rebels and end the nation's civil war this year.

The Tamil Tigers have been fighting since 1983 to create an independent homeland for ethnic minority Tamils, who have suffered decades of marginalization by governments controlled by the Sinhalese majority. The conflict has killed more than 70,000 people.

Human rights groups have warned that casualties among the hundreds of thousands of civilians living in the shrinking pocket of rebel territory were likely to mount as the government closed in on the insurgents.

The pro-rebel TamilNet Web site reported that seven civilians — including an 11-month-old baby — were killed Thursday in a government artillery assault on two rebel-held villages.

Nanayakkara said no civilians were living in the area of the attack.

Other rights groups have accused the government and its allies of trying to stifle dissent by vilifying opposition journalists as unpatriotic rebel sympathizers and creating an atmosphere that encourages violence against them.

On Thursday, Lasantha Wickramatunga, editor of the Sunday Leader newspaper, was killed by gunmen on motorcycle. He died in a hospital hours later. Police spokesman Ranjith Gunasekara said no one has been arrested for the killing.

The U.S. State Department condemned the attack, calling it a "shocking blow to independent media in Sri Lanka."

"The United States is deeply concerned that such attacks undermine efforts to build a united and democratic Sri Lanka where the rights of all people are protected," spokesman David A. Wood said in a statement released Friday.

Wickramatunga's newspaper was critical of President Mahinda Rajapaksa's government, accusing it of widespread corruption.