The U.N. secretary-general for the first time called Monday for the situation in Syria to be referred to the International Criminal Court, as world leaders including President Barack Obama and Russian President Vladimir Putin were addressing a global gathering with conflict at center stage.

Ban Ki-moon's state of the world address to leaders from the U.N.'s 193 member states came shortly before Obama, Putin, Chinese President Xi Jinping and Iranian President Hassan Rouhani were to speak to the U.N. General Assembly in the morning session alone.

The U.N. chief insisted on a political solution to the conflict in Syria, now well into its fifth year with more than a quarter of a million people killed.

Ban said five countries "hold the key" to a political solution to Syria: Russia, the U.S., Saudi Arabia, Turkey and Iran. He said "innocent Syrians pay the price of more barrel bombs and terrorism" and there must be no impunity for "atrocious" crimes.

The Syrian conflict is "driven by regional powers and rivalries," Ban said. On the sidelines of this week's meeting, leaders and diplomats from the major players are trying to address them.

Other crises at the center of discussions include the related refugee and migrant crisis, the largest since the upheaval of World War II.

Ban warned that resources to address these crises are dangerously low. "The global humanitarian system is not broken; it is broke," he said. The U.N. has just half of what it needs to help people in Iraq, South Sudan and Yemen, and just a third of what's needed for Syria.

The U.N. chief, in unusually hard-hitting words, also urged the world to unite against the "blatant brutality" of extremist groups including the Islamic State. He blamed "proxy battles of others" for driving the fighting in Yemen, and he warned against "the dangerous drift" in resolving the Israeli-Palestinian conflict, saying it is essential for the international community to pressure both sides to re-engage.

Monday's address by Putin, who has showed up the U.N. gathering for the first time in a decade, was one of the most highly anticipated. The Russian president also was set to meet Obama on the sidelines Monday afternoon.

Others set to speak Monday included French President Francois Hollande and Cuban President Raul Castro, who also has a meeting planned with Obama.

Some, including Obama, Xi and Hollande, already addressed the General Assembly over the weekend during a separate global summit on sweeping new U.N. development goals for the next 15 years.