Suspected Tamil Tiger guerrillas set off a powerful blast in Sri Lanka's main commercial hub on Saturday, wounding 45 people, officials said, as government forces kept up a major offensive against the rebels.

A package left near a fruit vendor exploded in Pettah, the commercial centre of the capital, police said, adding that the blast was also a short distance from the main bus station and the railway terminal.

"We received 45 people injured in the blast," a director of the Colombo National Hospital, Anil Jasinghe, said.

"There are four women and two children among the wounded. Only two or three need surgery, others have light to moderate injuries," he said.

There was no immediate claim of responsibility for the attack.

But President Mahinda Rajapakse's office said the separatist Liberation Tigers of Tamil Eelam (LTTE) guerrillas were believed to have carried out the bombing.

"Several recent plots by the LTTE to carry out bomb attacks targeting government installations, public transport and civilians have been foiled following information of such terrorist plots being revealed by LTTE cadres who were arrested by the police," the president's office said in a statement.

Police cordoned off the bustling Pettah commercial area following the bombing, which came as government forces continued to pound Tamil Tiger targets in the north of the island.

The Tigers have also been blamed for a string of bomb attacks against public transport and other blasts in recent months as the military intensified pressure on the rebels' defacto mini-state in the north.

The attack came as the defence ministry said another 18 guerrillas and a government soldier were killed in fresh fighting on Friday.

The latest fighting brought the number of rebels killed by troops since January to 6,185, according to government figures. The government says 582 of its soldiers have died in the same period.

Saturday's attack came a day after the rebels accused government forces of setting off a roadside bomb and killing two civilians inside guerrilla-held territory.

A man and a child died when their motorcycle was caught up in the bomb attack at Nedunkerni in the vast Wanni region on Thursday evening, the LTTE said in a statement.

The military routinely denies that it carries out attacks against civilians inside Tamil Tiger-controlled areas.

It is impossible to verify independently casualty figures as the government bars access to the front-lines by journalists and rights groups.

UN aid agencies say nearly 135,000 people have been driven from their homes due to fierce fighting in the past two months.

Sri Lankan troops have been pushing deeper into rebel-held territory as they try to dismantle the LTTE's northern stronghold. They ejected the guerrillas from the east of the island in July 2007.

Tens of thousands have died on both sides since the LTTE launched a separatist campaign in 1972 for a homeland for minority Tamils in the island's north and east.