COLOMBO (AFP) - Sri Lankan ground troops killed at least 16 Tamil Tiger rebels while air force fighter jets bombed guerrilla targets in the island's north, the defence ministry said on Saturday.

The air sorties were carried out over the rebel-held Iranamadu area late on Friday, the ministry said in a statement.

The offensive came hours after a suicide bomber, riding a motorbike, rammed into a police bus in the capital of Colombo, killing 11 people and wounding more than 80.

Troops advancing into rebel-held areas killed at least 16 guerrillas in Vavuniya on Friday, the statement said. It did not give details about any casualties suffered by government forces.

Meanwhile, 16 civilians were wounded in a hand grenade attack Saturday in Vavuniya, 260 kilometres (162 miles) north of Colombo, the ministry said, blaming the attack on the Tamil Tigers.

Police said the grenade was hurled towards a vehicle carrying police commandos but missed its target and exploded on the road.

There was no immediate comment from the Tamil Tigers.

Friday's fighting brought the number of rebels killed by security forces since the start of the year to 3,716 while 276 soldiers have died in combat during the same period, according to ministry figures.

Casualty numbers cannot be independently verified as Colombo bars journalists and rights groups from travelling to embattled areas.

The government said it had opened investigations into Friday's attack in Colombo's heavily-guarded Fort commercial area.

The area is home to the official residence of President Mahinda Rajapakse, the five-star Hilton and Galadari Hotels and the twin-tower World Trade Centre office complex -- a previous target of the Tiger rebels.

Friday's bomb was the third suicide attack in a week.

On the eve of May 10 provincial elections, a bomb ripped through a crowded cafe in eastern Ampara town, killing 12 people and wounding at least 36. Also, hours before polling started, a Tiger suicide diver sank a navy cargo ship docked at the eastern port of Trincomalee.

Tens of thousands of people have died since the Tigers launched an armed struggle in 1972 to carve out a separate homeland for minority Tamils in the Sinhalese-majority island's north and east.

Rajapakse on Friday denounced the bus attack as "savagery" and said it reinforced his decision in January to pull out of a truce with the Tigers and step up a military drive against the rebels' mini-state in the north.

Colombo has poured a record 1.5 billion dollars into the war effort this year, hoping for a quick end to the bloody conflict.

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