Colombo  - Security in Sri Lanka's capital was tightened after a bomb exploded inside a bus Tuesday minutes after it was detected and passengers got off, police said.

Despite the evacuation of the bus, four people were injured, police said. The explosion occurred in the centre of Colombo and caused extensive damage to the bus.

Police said the conductor of the state-run bus found the unattended parcel and evicted some 60 passengers before the bomb exploded.

Police spokesman Senior Superintendent of Police Ranjith Gunasekara blamed the Tamil rebels of the Liberation Tigers of Tamil Eelam (LTTE) for the attack.

Police and the defence ministry urged bus passengers and railway commuters to be vigilant as rebels may carry out attacks to distract the attention of security forces currently engaged in military operations in the northern part of the country.

UN aid workers deployed in strife-torn north on Tuesday completed the pullout of their staff in keeping with a government directive, as fighting between security forces and Tamil rebels intensified, officials said Tuesday.

Thirty-five UN aid workers, including 11 foreigners, were escorted by rebels out of the areas they controlled in the north, UN spokesman in Colombo Gordon Weiss said.

The aid workers will be operating from Vavuniya, 240 kilometres north of the capital, which is the last government-held town before entering rebel-controlled areas, he said.

International non-governmental organizations were also ordered to pull out of the conflict zone. Residents of the areas protested their withdrawal.

Government troops Tuesday moved further into rebel-controlled areas as the air force carried out airstrikes on rebel targets.

Air Force spokesman Wing Commander Janaka Nanayakkara said two airstrikes hit rebels in Kilinochchi, 320 kilometres north of Colombo.

But Tamil rebels claimed that airstrikes had hit civilians.

There was no immediate independent confirmation of either version.

An estimated 450,000 civilians live in the rebel-controlled areas and 200,000 of them are displaced, living in camps or temporary shelters. (dpa)