UNITED NATIONS — Secretary-General Ban Ki-moon demanded international action to stop the war in Syria, telling a somber gathering of world leaders Tuesday that the 18-month conflict had become “a regional calamity with global ramifications.”

In sharp contrast to the U.N. chief, President Barack Obama pledged U.S. support for Syrians trying to oust President Bashar Assad — “a dictator who massacres his own people.”

Opening the U.N. General Assembly’s annual ministerial meeting, Ban said in his state of the world speech that he was sounding the alarm about widespread insecurity and injustice, inequality and intolerance in many countries.

Putting the spotlight on Syria, the U.N. chief said “the international community should not look the other way as violence spirals out of control.”

“We must stop the violence and flows of arms to both sides, and set in motion a Syrian-led transition as soon as possible,” he said.

While Obama didn’t call for an end to the violence, he made no mention of arming the opposition and stressed the importance of ensuring “that what began with citizens demanding their rights does not end in a cycle of sectarian violence.”