Russia presides over a special Security Council meeting on countering terrorist threats that is bound to throw up sharp difference of views between Moscow and Washington over the crisis in Syria.
UN Secretary General Ban Ki-moon separately hosts a meeting aimed at agreeing on a global response to the exodus of hundreds of thousands of migrants to Europe, many of whom have fled the war in Syria.
The UN chief opened the General Assembly this week with a call to Europe "to do more" to ensure migrants are treated with dignity as countries shut down borders and police use water cannons to push back refugees.