One hospital official in the region said that zones the military had named as "safe areas" had been shelled during the ongoing operation.

The separatist group could not be reached for comment, but TamilNet, a website with links to the movement, said that at least 46 civilians had been killed since the bombings began in the region on Tuesday. 

The website also accused government forces of launching an attack on areas declared safe.

UN protests

Also on Thursday, the UN accused the Tamil Tigers of not allowing several staff members to leave Tamil territory.

"The LTTE's denial of safe passage is a clear abrogation of their obligations under international humanitarian law," the UN said.

The convoy, which arrived in a northern region known as the Vanni on January 16 for a humanitarian mission, had been due to leave on Thursday.

The Tigers have been steadily retreating since 2007 and have now been surrounded in the Mullaittivu district where they are thought to have many military bases.

Ashok Mehta, a retired general who served with an Indian Army peacekeeping force in Sri Lanka in the 1980s, told Al Jazeera: "The Tamil Tigers' aim is to live to fight another day and therefore they will try to relocate themselves."

The Sri Lankan government had set an objective of routing the Tigers by the end of 2008, and now Sri Lanka's army says it hopes to suppress any remaining resistance by April.

The separatists have been seeking independence for the island's minority Tamils since 1972, arguing that they are marginalised by the majority Sinhalese.