At least 52 civilians were killed in shelling in Sri Lanka's [Images] embattled northern Wanni region, where a hospital came under attack for the fourth straight day today -- this time by a cluster bomb, forcing authorities to evacuate patients from one of the last functioning medical facilities in the war zone.

"We have a confirmed report that 52 civilians have been killed in a shell attack at Suranthapuram (in Wanni region) on Tuesday evening," United Nations spokesman Gordon Weiss said, adding that it was not known who was responsible for the attack.

Today was the fourth consecutive day when the hospital was attacked, forcing authorities to evacuate patients from the building.

Sensing the precarious situation, the government had on Monday asked all civilians to enter the demarcated 'safety zone' as soon as possible, cautioning that otherwise their security could not be guaranteed.

The government, which is facing international pressure to ensure that the civilians trapped in the war zone are unharmed, also accused the LTTE [Images] of 'forcibly' holding the people back and using them as 'human shields'.

A 48-hour deadline set by government for the LTTE to allow civilians to leave the combat zone expired on Saturday night, after which the military continued operations to 'liberate' the civilians.

The latest attack on the hospital came as Sri Lanka marked its 61st Independence Day, with President Mahinda Rajapaksa declaring that the Tamil Tigers would be 'decisively' defeated in the next few days.

"I am confident that in a few days we will decisively defeat the terrorist force that many repeatedly kept saying was invincible," Rajapakse said in his speech during the 61st Independence day celebrations held amidst unprecedented security.

"Shadows of terrorism have almost been wiped out," Rajapaksa said as the Lankan forces have cornered the Tamil Tigers to a small patch of jungles in the country's north.

In a massive military operation, Sri Lankan forces have almost dismantled a mini-state created by the Tigers in northern Lanka, but an estimated 250,000 Tamil civilians trapped in the shrinking battle zone is posing problems for Colombo. The first causalities reported among those trapped people have triggered international pressure for their safety.

Lauding the performance of his forces, Rajapakse said "our heroic forces have given us the opportunity today to celebrate the anniversary of our Independence in a country freed, after many years, from the dark shadow of terrorism".