NEAR BAGHOUZ, Syria - US-led coalition warplanes struck Islamic State's last stronghold in eastern Syria and hundreds of civilians fled the besieged enclave on Monday as US-backed fighters pressed their campaign to seize it.Coalition jets roared overhead as columns of white smoke rose from the Islamic State-held Baghouz area a short distance from the Iraqi border, a Reuters witness said.The Syrian Democratic Forces (SDF), which have driven Islamic State (IS) from swathes of northern and eastern Syria with US-led coalition support, launched an offensive on Saturday to capture the enclave in Deir al-Zor province.The jihadists are putting up stiff resistance and had sought to counter-attack again on Monday morning, according to Mustafa Bali, head of the SDF media office. Around 1,500 civilians had fled the enclave on Monday, he added.SDF combatants watched as a column of at least 17 trucks filled with men, women and children left Baghouz along a dusty track into SDF-held territory. Women and children were crammed into the back of one of the trucks.Some of those fleeing identified themselves as Iraqis."It seems there are still many civilians inside Baghouz," Bali said. "We are compelled to go cautiously and accurately in this battle."On the outskirts of Baghouz, the people who had left stood in lines to be questioned by coalition and SDF forces apparently trying to identify whether any were jihadists.Ahead of launching the attack, the SDF said more than 20,000 civilians had left Baghouz in the preceding 10 days.