COLOMBO (AFP) - Sri Lankan troops entered the last urban area held by Tamil Tigers, the defence ministry said Saturday, as the UN repeated calls for the rebels to free civilians in the war zone. Government forces fought their way Friday into the small town of Puthukkudiriruppu, despite heavy resistance from the Liberation Tigers of Tamil Eelam (LTTE) and fought pitched battles elsewhere, the ministry said. Troops who entered the town "found the LTTE's high-tech satellite communication centre" and met "stiff resistance" as they advanced into built-up areas, the ministry statement said. Security forces seized a 1.5-kilometre (one-mile) stretch of rebel bunkers in an area east of Puthukkudiriruppu after heavy fighting on Saturday, the ministry said, adding that 13 rebels were killed in the fighting. At least nine more guerrillas were killed in Friday's fighting, the ministry said, but it did not say if security forces suffered any casualties. A military spokesman said troops destroyed 10 boats belonging to the Sea Tigers, the rebels' naval unit. The LTTE did not comment on the latest fighting, which has raised alarm over the safety of tens of thousands of civilians trapped in the war zone. The International Committee of the Red Cross said last month hundreds of civilians had perished in the fighting. Civilians "now face very great danger from fighting between the Sri Lankan government forces and the LTTE. And there is strong evidence the LTTE are preventing them from leaving," UN humanitarian chief John Holmes said. "I urge again all those with any influence on the positions of the LTTE to use that influence now to persuade them to let the civilian population go. There is no time to lose," he told the Security Council in New York on Friday. The UN as well as other governments have made similar appeals in the past few weeks. Estimates vary on the number of civilians trapped by the fighting. The government estimates the number at 70,000 and the UN says it could be about 200,000, while Tamil groups outside the country says it could be over 300,000. Holmes called on the government to "hold back from any final military battle in order to allow time for the civilian population to get out safely". Holmes said civilians were dying daily in the conflict zone, where government troops are battling to crush the Tigers' bid for an independent Tamil homeland in the majority-Sinhalese nation. Government forces have pushed the Tigers into a narrow piece of land and say they hope to completely crush the rebels by April - a victory that would bring an end to 37 years of armed conflict in the South Asian island nation. Sri Lanka's top defence official, Gotabhaya Rajapakse, said earlier this week the military push against the Tigers had slowed in a bid to ensure the safety of civilians whom he said were being held by the Tigers as human shields. Rajapakse, who is President Mahinda Rajapakse's younger brother, said the military had stopped using long-range weapons and air attacks against the remaining Tiger rebels who are known to mingle with civilians. The government says more than 36,000 civilians have escaped from the LTTE and sought shelter in government-held camps in the island's north.