The blast ripped through the aviary of the Dehiwala zoo, injuring the visitors who were rushed to the nearby hospital, the ministry said.

"Among the six people admitted (to hospital) was a person suffering from the shock caused by the explosion," the statement said.

The blast at the zoo, just outside the capital Colombo, caused panic among visitors, but police moved reinforcements to the area and urged calm.

The zoo has since been closed for visitors, officials said.

There was no immediate claim of responsibility, but the attack came a day after a powerful bomb ripped through a bus killing 20 passengers in the north-central town of Dambulla.

Sri Lankan police and security forces have maintained a high state of alert ahead of Monday's celebrations to mark the country's 60th anniversary of independence from Britain.

The latest attack came as the defence ministry said 46 combatants were killed in fresh fighting across Sri Lanka's embattled northern regions over the weekend.

Government troops backed by heavy artillery fire killed at least 44 suspected Tamil Tiger rebels on Saturday for the loss of two soldiers and recovered arms and ammunition, the defence ministry said in a statement.

There was no immediate comment from the Liberation Tigers of Tamil Eelam (LTTE) who have been fighting for autonomy for minority Tamils from the majority Sinhalese community.

Police sought public help to track down two men believed to have planted a parcel bomb on an overhead rack of the bus en route to the northern Buddhist pilgrimage town of Anuradhapura and detonated it by mobile phone.

"We have information that two people got off the bus before the bomb went off," police Deputy Inspector-General Kingsley Ekanayaka said. "We are trying to track them down. A search is underway."

Police said 68 injured people were being treated at three hospitals, adding that no foreign tourists were among the wounded.

The LTTE set off a powerful suicide truck bomb in the same area in October 2006, killing at least 116 sailors heading home on leave.

President Mahinda Rajapakse condemned the bombing and urged Sri Lankans not to be provoked by what he called the "savage attack" by the Tamil Tigers.

Meanwhile, the Tigers accused the military of blowing up a school bus inside rebel-held territory last month, which killed 18 people, including 11 children.

The government in January pulled out of a tattered peace pact with the rebels, who have been fighting for a separate Tamil homeland since 1972, a conflict that has left tens of thousands dead.