COLOMBO (AFP) - Sri Lankas cornered Tiger rebels put up stiff resistance despite calls for their surrender, the military said Thursday, as the UN ordered an urgent humanitarian mission to the troubled region. The army said the guerrillas were confined to a mere 10-12 square kilometres (around four square miles) of territory on the northeast coast, where thousands of civilians are still trapped by the fighting. As international concern mounted for the safety of between 15,000 and 20,000 civilians stuck in the conflict area, UN Secretary General Ban Ki-moon said he had ordered a humanitarian team to northern Sri Lanka. I intend to immediately dispatch a UN humanitarian team to the no-fire zone, he told reporters in Brussels. The purpose of this humanitarian team will be to first of all monitor the situation and support the humanitarian assistance and try to do whatever we can to protect the civilian population, the UN chief added. Sri Lankas military spokesman Brigadier Udaya Nanayakkara said the Tigers were using artillery and tanks in their last remaining bit of territory, an area the government had previously designated a no-fire zone. There are sporadic clashes but our priority is to get the civilians out. We can finish them off very quickly after the civilians get out of the way, he said. We can claim we have completely defeated the Tigers when we have captured the remaining area, he said. The defence ministry, meanwhile, said guerrilla resistance was dwindling. The rebels have been repeatedly accused of using civilians as human shields. Around 100,000 people have managed to escape rebel-held territory this week. President Mahinda Rajapakse has told the rebels to give up, but has ruled out any amnesty for rebel leader Velupillai Prabhakaran, who has led a ruthless decades-long battle for a separate Tamil homeland. The UN Security Council president also said on Wednesday that the Liberation Tigers of Tamil Eelam (LTTE) must now give up. There was no immediate comment from the rebels, but a pro-rebel website accused government forces of shelling the remaining rebel-held areas and killing and wounding more civilians, a charge denied by the military. The Sri Lankan army has made major advances in recent months, beating back the guerrillas, who at one time controlled more than a third of the island. The government insists the rebels are now all but finished. State television has shown thousands of people waiting for food; desperate civilians trudging through waist-deep water to get to safety and a young woman giving birth on a bus carrying displaced civilians away from the war zone. The UNs top humanitarian official in Sri Lanka said the situation was disastrous. I saw infants with dysentery, malnourished children and women, untended wounds, and people dressed in the ragged clothing theyve been wearing for months, said Neil Buhne, UNs Humanitarian Coordinator in Sri Lanka. We need funds for all the basics like food, medicine, water, sanitation, nutrition, shelter, and clothing, Buhne said after managing to visit the northern town of Vavuniya, where over 80,000 people are stuck in makeshift camps. Above all, however, the Sri Lankan government has been told by the UNs Ban that aid workers must be given full access to the north. The European Union and United States also pressed the same demand again on Thursday. Sri Lanka has blocked most aid agencies and has herded escaping Tamil civilians into closely-guarded internment camps so it can weed out suspected rebels. Sri Lankas Foreign Minister Rohitha Bogollagama said he welcomed international aid, but signalled access to the north for aid workers and journalists would remain subject to restrictions. Tamils around the world have demonstrated against the military campaign. In neighbouring India on Thursday, Tamil Nadu state came to a halt in protest over the conflict in Sri Lanka.