COLOMBO (AFP) - Sri Lankas military readied Monday for a final assault on Tamil Tigers boxed into a strip of jungle with thousands of trapped civilians, after urging the rebels to surrender or face annihilation. Security forces were moving ahead after killing at least 525 Tiger rebels in four days of fierce fighting in the northeast of the island that ended on Sunday, military officials said. Troops on Monday recovered 70 bodies of Tigers from Puthukkudiriruppu township which was brought under control of the security forces on Sunday and raised the total number of guerrillas killed in the past four days to 525, a military official said. He said the guerrillas were now confined to a 20 square kilometre (eight square mile) coastal area which the military has designated a safe zone for tens of thousands of civilians trapped in the conflict area. They continue to use civilians as a human shield and attack security forces from the safe zone, military spokesman brigadier Udaya Nanayakkara said. Sporadic clashes were reported from the region on Monday. Army chief Lieutenant General Sarath Fonseka told US ambassador Robert Blake that he believed Tamil Tiger supremo Velupillai Prabhakaran may still be hiding in the area. An exodus of trapped civilians could be expected at any moment, Fonseka told Blake according to an army statement. In the past four months, some 65,000 people have managed to flee the war zone and find shelter in government-run camps. The United Nations and other foreign aid organisations say as many as 150,000 civilians may still be trapped, although the Sri Lankan government insists the figure is less than half that. President Mahinda Rajapakse on Sunday announced that the military was on the verge of wiping out the Tigers after decades of bitter fighting. The option for the Tiger leadership is to lay down arms and surrender and save the lives of the remaining cadres, the president said. Rajapakse, who is also the commander in chief of the armed forces, has rejected international calls for a truce which the government has argued would only serve to allow the rebels a breathing space to regroup. The Tigers have been encircled for months and the military says it would have finished them off much quicker but for the trapped civilians. In a statement Monday on its website, the presidents office said the armys next move would be a humanitarian exercise to free the non-combatants. The UN human rights chief, Navi Pillay, has said more than 2,800 civilians have been killed in shelling since January 20 a charge denied by the Sri Lankan authorities. At the height of the Tigers power in the mid-1990s, they controlled more than a third of the total land mass of Sri Lanka in their fight for an independent Tamil homeland. Sri Lanka, which pulled out of a Norwegian-arranged truce in January last year, stepped up attacks in a determined push to finish off the Tigers, a success that would end Asias longest running ethnic conflict.