UNITED NATIONS — The United Nations General Assembly opens on Monday with all eyes on the war in Syria and the twin crises it has helped spawn: the unyielding spread of the Islamic State in Syria and Iraq, and the surge of refugees from the region into Europe.

Leaders of the world’s most powerful nations are due to speak in the morning at the 70th annual General Assembly debate, including President Obama, followed by Presidents Xi Jinping of China, Vladimir V. Putin of Russia and François Hollande of France.

The more the conflict endures, the more it fragments, the more it radicalizes, the more intractable.

President Hassan Rouhani of Iran, which is among Syria’s most dogged allies, is scheduled to step up to the lectern in the morning as well, along with King Abdullah II of Jordan, whose country is swelling with 630,000 registered Syrian refugees.