Russia will brief the United Nations Security Council on Tuesday about the results of the Tehran summit it held with Iran and Turkey on the fate of Syria's Idlib province, diplomats said. The council will meet at Russia's request at 11:00 am (2030 IST) to hear the briefing as Syrian forces, backed by Russia and Iran, are preparing for an all-out military assault on the rebel-held province. The United Nations has warned of a major humanitarian catastrophe if the offensive takes place in the enclave where some three million people live - about half of whom have already been displaced in the seven-year war. On Friday, the presidents of Iran, Russia and Turkey failed to agree during the Tehran summit on how to stave off military action on the northwestern province. At a council meeting also on Friday, Russian Ambassador Vassily Nebenzia defended the military plans for Idlib, saying armed groups linked to Al-Qaeda were "a legitimate target for liquidation." Britain, France and the United States fear mass casualties among civilians during the assault on Syria's last major rebel bastion. The Russian mission to the United Nations said in an email seen by AFP that the public meeting would serve to "brief council members on the results" of the Tehran summit of the so-called Astana guarantors. The three countries last year set up the Astana process, a negotiating track to end Syria's war that has largely eclipsed the UN-led peace process.