Tamil rebel planes bombed government positions Tuesday in northern Sri Lanka in their second-ever airstrike. The military said six soldiers were killed but that the aircraft were turned back before reaching a key base.

The attack came amid rising fears that the government may be preparing a major assault on rebels in the north, and followed a bomb blast Monday aboard a bus that killed five passengers and wounded 35 near the frontier between government- and rebel-controlled areas.

The Tamil Tiger rebels said two airplanes carried out Tuesday's air attack on the government's Palaly military base in Jaffna peninsula, and that main armory and storage facilities were struck.

Lt. Col. Upali Rajapakse, a senior military official, said only one light aircraft was involved in the attack and that troops opened fire on it, forcing it to turn back before it reached the Palay base. But the plane dropped two bombs on nearby bunkers while rebels shelled the area.

Six soldiers were killed by the airstrike and shelling, Rajapakse said.

"Our pilots have confirmed that they hit the main armory and storage facilities in Palaly. Pilots have seen fire," Tiger spokesman Rasiah Ilanthirayan said by telephone from the rebel stronghold of Kilinochchi. "It may have caused damage to their command chain located within the base."

Palaly is a strategic military base with an air strip and functions as the headquarters for military operations against Tiger guerrillas in the north. It is also the supply base for tens of thousands of soldiers stationed in the ethnic Tamil-majority Jaffna peninsula.

It was the second rebel airstrike since the Tigers started their campaign for a homeland for the country's Tamil minority in 1983. The first was launched last month, when at least one rebel propeller plane bombed a Sri Lankan air force base outside the capital, Colombo, killing three airmen and wounding 16.

On Monday, Sri Lankan officials ordered Norway's ambassador, who is trying to mediate a resumption of peace negotiations, to cancel a trip to the Tamil Tiger rebels' northern strongholds for security reasons, leading to speculation that a major government assault may be planned.

Earlier Monday, the government's air force bombed a rebel sea base and a moving column of guerrillas separately in the country's north and east, according to the military.

Also Monday, a bomb ripped through a long-distance bus as it traveled near the town of Vavuniya, killing five passengers and wounding 35, the Defense Ministry said. The ministry blamed the attack on the Tamil Tiger rebels.

Vavuniya, about 130 miles north of the capital, Colombo, is where the government has its last key garrison before the start of rebel-controled territory.

A cease-fire mediated by Norway in 2002 brought hopes of peace for a few years, but violence over the past 18 months has killed 4,000 people, taking the death toll from more than two decades of war past 69,000.