"I want to express my deepest concerns about the escalation of conflict in eastern Syria. It is absolutely essential to de-escalate," he told reporters in Copenhagen.
Turkey pressed its military offensive against U.S.-allied Kurdish fighters in northeast Syria on Thursday, shelling towns and bombing targets from the air in an operation that has forced thousands of people to flee their homes.
"Military operations must always respect the United Nation's chapter and international humanitarian law, and I am worried with the humanitarian concerns that exist in relation to not only casualties but also the displacement that is taking place," Guterres said.