Backed by Russian air power, Syrian regime forces have advanced on Idlib at a rapid clip since last week.
An assault on rebel-held northwest Syria by regime forces has pushed some 700,000 people to flee toward the Turkish border and raised the spectre of an international crisis, US Special Envoy for Syria James Jeffrey said on Thursday.
Backed by Russian air power, regime forces have advanced on Idlib at a rapid clip since last week, taking back dozens of towns and upending a region where millions have taken refuge since the start of Syria's nearly nine-year war.
The campaign has ratcheted up tensions between Moscow and Ankara. Turkey fears a fresh wave of migrants piling across its border and has a dozen observation posts in Idlib, part of a de-escalation agreement it says Russia is now violating.
Speaking at an online news briefing, Jeffrey said that in the last three days Syrian regime and Russian warplanes had hit Idlib with 200 air strikes "mainly against civilians", and that several Turkish observation posts had been "cut off" by the regime advance.
There are "massive movements of troop s pushing back hundreds of square kilometres and setting - I think now - 700,000 people who are already internally displaced on the move once again towards the Turkish border, which will then create an international crisis," said Jeffrey.
Turkey already hosts more than 3.5 million Syrian refugees.
Moscow and Damascus say they are fighting militants who have stepped up attacks on civilians in Aleppo in northern Syria, but rights groups and rescue workers say air strikes have demolished hospitals, schools and hit other civilian areas.
In a significant milestone for Bashar al Assad's stated drive to reclaim all of Syria, regime forces on Tuesday took Idlib's second biggest city, Maarat al Numan, an urban centre that straddles the M5 international highway linking the capital Damascus to Aleppo and considered vital for trade.
Smoke still billowed from some buildings in t he city on Thursday while the demolished exteriors of others tumbled onto streets emptied entirely of civilians.
Syria's war-torn economy has plunged deeper into crisis in recent months, with a rapidly weakening currency driving up inflation and aggravating hardship for Syrians struggling to afford basic goods.
"What they're doing is using Idlib as a sort of distraction that allows time to keep their loyalist constituencies on a war footing," said Yezid Sayigh, a senior fellow at the Carnegie Middle East Center in Beirut.
"This is a regime that is unable to demonstrate effective control and revive economic activity, revive markets, and hold up the lira," said Sayigh.
A UN report on Thursday described increasingly bleak conditions for Syrians in urgent need of shelter and food while fleeing bombardment.
"Whole towns have emptied as an increasing number of civilians flee northward to areas deemed safer, but which at the same time are rapidly shrinking, as territorial advances against opposition forces continue," said David Swanson, the UN regional spokesperson for the Syria crisis in Amman.
"This latest wave of displacement underscores the fact that this war, now almost nine years old, is far from over," said Swanson.