The United Nations' top humanitarian official begins a three-day mission in Sri Lanka on Sunday, seeking access to some 50,000 civilians trapped and reportedly starving in the country's sealed-off war zone. U.N. Under-Secretary General for Humanitarian Affairs John Holmes said he hopes to persuade the Sri Lankan government to suspend its assault and allow a humanitarian team into the conflict zone, where the civilians are caught between government forces and the beleaguered Tamil Tiger rebels. Reports of chaos and suffering in the northern war zone have increased in recent days as the Sri Lankan military pushed forward with its offensive to destroy the separatist insurgency and end the Indian Ocean island nation's bloody quarter-century civil war. Aid workers have been barred from the region since fighting escalated last year. Mr. Holmes said Saturday that the trapped civilians are suffering from a very high'' casualty rate, and from lack of food, clean water and medical supplies. The U.N. says nearly 6,500 civilians have been killed in the fighting over the past three months. The situation of those people is very dire and that's why we need to find a way to stop the fighting and get them out of there so we can look after them properly,'' he told Associated Press Television News in Thailand en route to Sri Lanka. More than 100,000 civilians have fled the tiny coastal strip still under rebel control since Monday, flooding hospitals in the north and overwhelming government-run displacement camps, according to aid workers. The U.N. says another 50,000 civilians remain trapped in the war zone. But the rebels say the number of trapped civilians is three times that estimate. The Tamil Tigers said in a statement that food stocks in the region had dwindled. The rebels called on the U.N. and the international community to ensure that food supplies are swiftly sent to the area. The rebels said that the civilians in the war zone are faced with serious crisis due to the deliberate denial of food and other humanitarian supplies by the Sri Lankan government.'' The dwindling stocks coupled with the deliberate withholding of fresh supplies has made starvation imminent,'' the statement said. The government insists that it has sent food and medicines and accuses the rebels of holding the civilians as human shields to protect reclusive rebel leader Velupillai Prabhakaran from the advancing troops. It is not possible to verify the claims because the government has barred independent journalists from the war zone, arguing that it is too dangerous for them to work. The rebels, listed as a terrorist group by many Western nations, have been fighting since 1983 for an ethnic Tamil state in the north and east after decades of marginalization by governments dominated by the Sinhalese majority. After more than three years of intense fighting, the military stands on the verge of crushing the group. International pressure on Sri Lanka has grown in recent days, but the government has brushed off calls for a cease-fire. Mr. Holmes said in Thailand that the U.N. hopes to get a preliminary team in the conflict area in the next two or three days, but that depends on there being some kind of a pause in the fighting to allow that to happen.'' The government has been reluctant to suspend the fighting for fear the rebels will exploit any lull in the conflict, he said. Mr. Holmes also said he would press for the immediate release of 15 U.N. staffers who have been detained in government camps since escaping from the conflict zone.