COLOMBO: Thousands of civilians have fled Sri Lanka’s northern war zone in recent days, crossing the front lines amid fierce fighting as the army closed in on the rebels’ last stronghold, the military said on Friday. The government says it hopes the exodus represents the start of a mass flight that will remove hundreds of thousands of bystanders from harm’s way as the military tries to crush the Tamil Tigers’ decades-old insurgency. Aid workers and diplomats have expressed growing concern for the fate of the civilians trapped in what remains of rebel-controlled territory in the northeast after months of fighting. While the military has so far avoided large-scale civilian deaths, there have been increasing reports of civilian casualties. Civilians have largely ignored past government appeals for them to cross into military-held areas. However, with the rebel-held area shrinking and nowhere left to run, families now have begun fleeing in large numbers, the military said. On Thursday alone, 1,069 civilians fled into government territory, the military said. The day before, 874 crossed over. A total of 2,735 fled in the first 15 days of 2009, dwarfing the 1,600 who left in all of 2008. “It has started, and day by day it is increasing,’’ military spokesman Brig Udaya Nanayakkara said. The military has vowed to destroy the rebel group and in recent weeks has captured the Tamil Tigers’ de facto capital of Kilinochchi and boxed the insurgents into a small pocket of territory in the northeast. On Friday, the military said it had captured a sixth airstrip used by the rebels’ tiny airborne unit.