Some 200 families are trapped in a shrinking area of Syria controlled by Islamic State, whose forces are stopping many from fleeing, U.N. human rights chief Michelle Bachelet said on Tuesday, calling for the families' safe passage.The US-backed Syrian Democratic Forces (SDF) are on the brink of defeating IS in its last pocket in eastern Syria, the village of Baghouz, where it estimates a few hundred Islamic State fighters and about 2,000 civilians are under siege.Many of the families .".. continue to be subjected to intensified air and ground-based strikes by the US-led Coalition forces and their SDF allies on the ground," Bachelet said in a statement."We understand that ISIL appears to be preventing some of them if not all of them from leaving. So that's potentially a war crime on the part of ISIL," her spokesman Rupert Colville told a news briefing.The SDF attacking Islamic State have an obligation under international law to take all precautions to protect civilians who are mixed in with the foreign fighters, he said.