ANKARA, Turkey — Syrian doctors and humanitarian workers voiced growing alarm at the plight of civilians in northwestern Syria this week, as fighting intensified in the nation’s last rebel-held province and hundreds of thousands of people fled north toward the Turkish border.

Turkey has closed that border, after eight years of civil war in Syria spurred more than three million people to seek safety in its boundaries. Now, refugees are camping in open fields and in desperate conditions, moving repeatedly as Syrian and Russian forces have escalated artillery and aerial bombardments in a concerted attempt to advance into the province, Idlib.

Over a quarter-million people have been displaced in the past month and 160 people have been confirmed killed, the United Nations said on Thursday, warning of an impending disaster if the violence is not stopped. Officials say the actual number of dead is much higher than 160.

Video footage of the bombardments, and of dead and injured children being pulled from the wreckage of homes, has flooded social media. In one, a small girl screams as she tugs at the arm of her brother, trapped under rubble. In another, a teenager pulled from a crushed building is told that his brother, lifeless beside him, is sleeping.