Syrians escaping violence in Idlib have arrived in villages near the Turkish border in droves, the U.N. humanitarian affairs office (OCHA) said Monday, fearing Damascus will launch a full-blown assault on the rebel stronghold.

More than 30,000 people have fled their homes in northwest Syria since the army and allied forces resumed bombardment last week, the office said, adding that a military offensive could trigger the worst humanitarian catastrophe of the 21st century.

"What happened was destruction all over, burning — something you can't describe," said Ali al-Mheymid, 50, who escaped the village of Sarjah in Idlib province with his family last week.