United States President Donald Trump has warned the Syrian government and its Russian and Iranian allies against “recklessly” attacking rebel-held Idlib province.
In a social media message, he warned of “a grave humanitarian mistake” in which hundreds of thousands of people could be killed.
Syrian government forces are said to be preparing a huge offensive on the last major rebel stronghold in the country.
United Nations says such a campaign could have disastrous consequences for thousands of civilians.
The US state department also warned on Monday that Washington would respond to any chemical attacks by the Syrian government or its allies.
US ambassador to the UN Nikki Haley said: “All eyes on the actions of Assad, Russia, and Iran in Idlib. #NoChemicalWeapons”.
The UN is desperate to avoid the deaths of civilians seen recently in other parts of Syria such as Aleppo, Raqqa or the Eastern Ghouta, and its diplomats are appealing for all sides to show restraint.
Idlib has been a haven for rebels and their families evacuated out of areas won back by the government, but there is no obvious place for them to move to within Syria if they abandon the province now.