U.S. President Barack Obama has called on global leaders to “speak out forcefully against violence and extremism,” saying the world can only make progress by pursuing tolerance and freedom.

Mr. Obama spent much of his speech Tuesday at the United Nations General Assembly on the weeks of violent protests sparked by an anti-Muslim film made in the U.S.

He called on delegates to marginalize those who use hatred as a political weapon.

“If we are serious about upholding these ideals, it will not be enough to put more guards in front of an Embassy; or to put out statements of regret, and wait for the outrage to pass. If we are serious about those ideals, we must speak honestly about the deeper causes of this crisis.”

Mr. Obama called the video crude and disgusting, but said it did not justify the spilling of any innocent blood — including U.S. Ambassador to Libya Chris Stevens, who was killed in an attack on the U.S. consulate in Benghazi.

The U.S. president also renewed calls for an end to the regime of Syrian President Bashar al-Assad.

“In Syria, the future must not belong to a dictator who massacres his people. If there is a cause that cries out for protest in the world today, it is a regime that tortures children and shoots rockets at apartment buildings.”

Mr. Obama also sent another warning to Iran, saying the United States “will do what we must” to prevent Tehran from getting a nuclear weapon.

“Make no mistake: a nuclear-armed Iran is not a challenge that can be contained. It would threaten the elimination of Israel, the security of Gulf nations, and the stability of the global economy.”

U.N. Secretary General Ban Ki-moon addressed the General Assembly earlier Tuesday, calling on member-nations to do more to ease the crisis in Syria.

He called the conflict there “a regional calamity with global ramifications” and accused both the government of Syrian President Bashar al-Assad and the opposition of crimes against humanity.

The world is deadlocked on how to deal with that conflict, which has killed more than 20,000 people, mostly civilians. Russia and China have vetoed tough sanctions against the Syrian government in the Security Council, and neither the United States nor its allies have thus far called for direct military intervention.

Mr. Ban also admonished world leaders for their reaction to the violent protests that have gripped parts of the Middle East and Asia. He said leaders must do more to defuse tensions, saying “'too many people are willing to take small flames and…turn them into a bonfire.”

The secretary general deplored the “dangerous impasse” between Israel and the Palestinians that could close the door on a two-state solution, and criticized the rhetoric between Israel and Iran over Tehran's nuclear program.

More than 120 world leaders are set to attend the General Assembly meeting to discuss and debate wars, political crises and humanitarian concerns.