US President Donald Trump has warned the Syrian government and its Russian and Iranian allies against “recklessly” attacking rebel-held Idlib province.
In a tweet 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.
The UN says such a campaign could have disastrous consequences for the almost three million civilians living there.
The US state department also warned on Monday that Washington would respond to any chemical attacks by the Syrian government or its allies.