The United States accused on Friday Russia of “defending” the Syrian regime’s imminent offensive on the opposition-held northwestern province of Idlib. US Secretary of State Mike Pompeo tweeted that his counterpart "Sergei Lavrov is defending Syrian and Russian assault on #Idlib. The US 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." Regime forces have been massing around the province of Idlib, which borders on Turkey, for days. 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."