RAQQA, Syria (Reuters) - Air raids by U.S. coalition warplanes have intensified in recent days as Kurdish and Arab militias seek to drive surrounded Islamic State militants from their last strongholds in Syria s Raqqa   but the toll on civilians has been severe.  Hundreds of civilians fled the city on Thursday, many wounded and malnourished after being trapped for months by fighting between Islamic State and the U.S.-backed Syrian Democratic Forces (SDF).  Those who fled said the sheer intensity of the bombardment appeared to have made militants shift their positions, retreat or hide underground, giving civilians a window in which to escape. Abdullah Ali s burns were still raw from an air strike that brought down his building and killed his entire family a week ago in central Raqqa.  My wife, mum, dad, all 14 people in my family were killed. Their bodies are trapped under rubble,  the 24-year-old said, sitting outside a mosque on Raqqa s outskirts. Ali s neighbor Abdo Hussein said more than 50 people were in the building when the air strike hit. Just a handful survived and 13 bodies had been pulled out, he said. The offensive to drive Islamic State out of Raqqa, its de facto Syrian capital which it seized in 2014, has long outlasted initial predictions by SDF officials who said ahead of a final assault in June that it could take just weeks. The SDF said last week the city could be declared captured in the coming days. There are still several hundred militants in the city and thousands of residents, the coalition says, many of them believed to be held hostage by IS in a hospital and nearby stadium.     People had tried to escape before but were shot at by Daesh (Islamic State). I even saw them kill a two-year-old child,  Um Moussa, 38, said, sitting inside the mosque.  This morning they didn t seem to be around, or weren t firing.  My son saw hordes of people leaving so we decided to go for it. I d been sleeping fully dressed   we were ready to flee at the first chance,  she said, wearing black robes and a face veil required under Islamic State s strict laws. But weakening the militants with air power has come at a high cost in civilian lives, she and others said.  Yesterday four entire families were killed in our area. It s strike after strike.  All those who escaped on Thursday came from a district near the stadium. They said many buildings had been hit as Islamic State fighters fired from them.  Each building has dozens of civilians in it, so of course many have died,  said Hussein. Air strikes were precise, often taking out a single building without damaging those next door, but militants often managed to leave before the missiles hit, he said. The coalition says it takes great pains to avoid causing civilian casualties and investigates all reports that it has done so.  Residents described miserable living conditions and lack of food, water and medical aid with the remaining areas Islamic State controls completely cut off.  Daesh have clinics but it s to treat their fighters, not us,  Hussein said. Umm Mousa said she, her husband and eight children had slept in their cellar by night to hide from the bombardment and the militants, but that Islamic State was now deliberately avoiding interaction with residents.  They re suspicious, scared, in case any of the civilian population are informers   they re keeping their distance a bit now,  she said. Local Islamic State fighters have been surrendering in recent weeks, the U.S. coalition and residents say. Outside the mosque, several blindfolded men were brought into an SDF headquarters for questioning. Islamic State foreign fighters are expected to fight to the death, however.  The foreigners, those are the hard core,  said one escaped resident, Ahmed Faraj.  They won t give up.  
Families fleeing Syria's Raqqa say air strikes bring heavy toll