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 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.
Up to three million people in the region are bracing for imminent attack by Russian forces who sense victory in one of the last parts of Syria still out of regime control. On Friday the Russian navy manoeuvred in the nearby Mediterranean, while other nations also took positions; the Turkish army sent an armoured convoy deep into Syria, Iranian-backed militias mobilised to the south and the Syrian army was placed on high alert.
The northern province and surrounding areas are the last major enclave held by insurgents fighting Assad, who has been backed by both Russian and Iranian forces in Syria’s seven-year-old civil war.
Trump has sought better relations with Russia since taking office in 2017 but the US has been unable to rein in Moscow’s military and diplomatic support for Assad.
The US secretary of state, Mike Pompeo, said on Friday that Washington viewed any government assault on Idlib as an escalation of Syria’s war, and the state department warned that Washington would respond to any chemical attack by Damascus.
The day before Pompeo’s warning, Lavrov told Western nations not to “play with fire” over Idlib and claimed the White Helmets, a group of unarmed volunteer rescue workers, were preparing to stage a chemical strike to blame on the Syrian government as a pretext for military action.
The Pentagon responded Friday, saying, “Russia has recently launched a concentrated disinformation campaign to discredit the United States and international partners and allies” and calling Lavrov’s allegation about preparations for a staged chemical strike “absurd.”
CNN reported Friday that US intelligence and military targeting experts have created a list of Syrian chemical weapons facilities that could be struck if Trump decided to order a new round of airstrikes in the country.
So far, a decision to take action has not been made, but one administration official with direct knowledge of the situation told CNN the military “could respond very quickly” if Syria launched a chemical weapons attack.
Earlier on Monday, Iran called for militants to be “cleaned out” of Idlib as it prepared for talks with Syria and Russia about confronting the last major enclave held by rebels opposed to Assad.