Sri Lanka ruled out a ceasefire with Tamil Tiger insurgents on Friday, despite growing reports of civilian casualties, as the rebel group insisted the estimated 250,000 civilians trapped in the northern war zone do not want to leave the group's protection. 

Sri Lankan President Mahinda Rajapaksa had urged the rebels to let the civilians leave the area by Saturday and guaranteed safe passage to all noncombatants from the area.

B. Nadesan, a political official with the Tigers, known formally as the Liberation Tigers of Tamil Eelam, told the BBC on Friday that the civilians did not want to end up in the hands of "their killers."

Nadesan was quoted as saying at least 28 people had been killed since the president's offer of safe passage on Friday.

The 25-year war of separation between the country's Singhalese Buddhist-dominated government and the Hindu Tamil Tigers is believed to be close to an end.

Sri Lankan security forces have recaptured most of the territory it lost to the Tigers over the years by unleashing intense air and ground attacks in recent months.

The government insisted Friday there would be no let up in its campaign to crush the rebels.

"We are determined not to have a ceasefire, and we are determined to eradicate terrorism in Sri Lanka," Human Rights and Disaster Management Minister Mahinda Samarasinghe told reporters.

'Grave breaches' by both sides: UN

Human rights groups have accused the Tigers of holding the civilians hostage and accused the military of launching heavy attacks in areas filled with noncombatants, including the government-declared "safe zone" in the north. The insurgents and the military deny the charges.

UN High Commissioner for Human Rights Navi Pillay said top UN officials were "seriously alarmed" over the fate of civilians in the north.

"It seems there may have been very grave breaches of human rights by both sides in the conflict, and it is imperative that we find out more about what exactly has been going on," she said.

The government's Samarasinghe disputed the figure, saying fewer than 120,000 civilians were in the war zone. He denied reports that more than 300 civilians were killed in recent fighting and accused the rebels of forcibly recruiting civilians, giving them two or three days of training and putting them on the front line as cannon fodder.

"We have not targeted civilians, and we will not target civilians," he said.

The Tamil Tigers have fought since 1983 to create a separate state for minority Tamils, who have suffered decades of marginalization at the hands of governments controlled by the Sinhalese majority.

More than 70,000 people have been killed in the civil war.

With files from the Associated Press