COLOMBO, Sri Lanka - Sri Lanka's president promised to safeguard civilians caught in the war between government and Tamil rebel forces as the UN warned Friday that the entire conflict zone is facing a massive food crisis.

The humanitarian crisis was building as the military continued its relentless offensive, which has almost routed the Tamil Tigers, virtually ending their 25-year war for a separate Tamil nation in this Sinhalese-majority country. On Friday, troops captured the headquarters of a rebel regiment, the military said.

President Mahinda Rajapaksa assured UN Secretary-General Ban Ki-moon in a 15-minute telephone conversation Thursday that the offensive "would be carried out without harassment to the civilian population," a statement from the president's office said.

But evidence has grown in recent days of mounting casualties among the estimated 250,000 civilians trapped in the shrinking sliver of land still controlled by the rebels. Hundreds fled the area on Friday, joining thousands who have escaped in the past few days.

Reports from the sealed war zone, known as Vanni, were spotty. But the top health official there said last week that 300 civilians had been killed, and the UN said at least 52 civilians were killed Tuesday.

Emilia Casella, a spokeswoman for the World Food Program in Geneva, told reporters that the entire population of the Vanni is facing a food crisis. Some 250,000 people there are completely dependent on humanitarian aid, but WFP has not been able to get a supply convoy into the conflict zone since Jan. 16, she said.

A convoy that was supposed to enter during a 4-hour "humanitarian window" Thursday could not go because the agency did not receive the necessary clearance from government officials, she said. The earliest they would be able to send in another convoy is next Thursday, she said.

"We don't have any more stocks to be distributed, and our staff are essentially hiding at the moment," Casella said. WFP has 16 staff and 81 dependents in the Vanni area.

Despite the growing concerns for the fate of civilians in the war zone, the government has rejected calls for a cease-fire to allow them to escape the fighting.

At least 600 civilians crossed the front lines into the government-controlled area Friday, said military spokesman Brig. Udaya Nanayakkara. He said a total of 1,637 civilians crossed Thursday.

Amnesty International called on both sides to declare a humanitarian cease-fire to allow civilians out and to let food, water and medical supplies be delivered to those who can't leave.

"A quarter of a million people are suffering without adequate food and shelter while shells rain down upon them. Most of those who have managed to escape the conflict have not received adequate hospital treatment," said Yolanda Foster, a researcher at the London-based rights group.

Amnesty also expressed concerns that the civilians who have already fled the war zone were being housed in military-run transit camps with little freedom of movement.

Meanwhile, troops captured the headquarters of the rebel group's Radha regiment in Vishwamadu area on Friday, Nanayakkara said.

The regiment is responsible for providing security to the Tamil Tiger leader Velupillai Prabhakaran and to the rebel group's tiny air force, now defunct. However, no rebels were captured as they fled on seeing the advancing troops, he said.

"Troops surrounded the area so fast that the (fleeing) terrorists couldn't even take their flag," he said.

The military captured the rebels' biggest sea base Thursday, effectively cutting off their main supply point.

Also Friday, some 150 demonstrators demonstrated outside the office of the International Committee of the Red Cross in the capital Colombo, apparently protesting its reports of civilian casualties.

Some among the protesters threw stones at the office, causing minor damage but no injury to staff, said Red Cross spokeswoman Sarasi Wijeratne.

Some 70,000 people have died in the Tamil conflict, which began in 1983 after years of marginalization of the Tamil minority by governments dominated by the Sinhalese majority.