US Secretary of State Mike Pompeo on Friday accused his Russian counterpart Sergei Lavrov of "defending the assault" by Syrian government forces on the last rebel stronghold in Idlib province.
"Sergey Lavrov is defending Syrian and Russian assault on #Idlib," Pompeo tweeted. "The Russians and Assad agreed not to permit this. The U.S sees this as an escalation of an already dangerous conflict."
He added: "The 3 million Syrians, who have already been forced out of their homes and are now in #Idlib, will suffer from this aggression. Not good. The world is watching."
Forces loyal to the Syrian President Bashar al-Assad have been massing around the province of Idlib, which bordes on Turkey, for days and seem ready to launch what could be the last major battle of the civil war that has torn Syria apart since 2011.
Turkey, Russia and fellow regime backer Iran all operate "observation points" in Idlib as part of a "de-escalation" deal agreed last year that was meant to reduce bloodshed in the province.
The prospect of a massive Russian-backed offensive in a province that is home to some three million people -- half of them already displaced from other parts of Syria -- has raised fears of a new humanitarian tragedy.
UN chief Antonio Guterres said Wednesday he was "deeply concerned about the growing risks of a humanitarian catastrophe in the event of a full-scale military operation in Idlib."
Lavrov hinted the assault may be imminent during a Wednesday press conference with his Saudi counterpart in Moscow.