United Nations Secretary-General Antonio Guterres on Thursday voiced “grave concern” about the escalation of fighting in northwest Syria and reiterated his call for a ceasefire. “In recent days I have warned repeatedly about the risk of a serious escalation of the hostilities in northwest Syria. I fear that with the events of the past 24 hours, we have reached that point,” he told reporters in New York Friday. The U.N. chief was speaking after 33 Turkish troops were killed Thursday in a Syrian government airstrike in Syria’s Idlib province, the deadliest day for the Turkish military since it entered the conflict in 2016. Russian-backed Syrian troops have been waging a fierce campaign to oust rebels from their last stronghold in Idlib. Guterres said a cease-fire is needed “before the situation gets entirely out of control.”