An assault on northwestern Syria's Idlib by regime forces in recent days has pushed some 700,000 people to flee toward the Turkish border, raising the specter of an international crisis, U.S. Special Envoy for Syria James Jeffrey said Thursday.

Backed by Russian air power, regime forces have rapidly advanced on Idlib since last week, upending an area where millions have taken refugee since the start of Syria's nearly nine-year war.

Jeffrey told a news briefing that Syrian regime and Russian warplanes had hit Idlib with 200 airstrikes "mainly against civilians" in the past three days.

He said the assault had set "700,000 people who are already internally displaced on the move once again toward the Turkish border, which will then create an international crisis."

President Recep Tayyip Erdoğan said Wednesday that Ankara was losing patience with the Idlib assault and would retaliate against any attack on its 12 observation posts in the area.

Turkey already hosts more than 3.5 million Syrian refugees and fears millions more could soon cross the border.