U.S. President Barack Obama is set to tell Iran the United States “will do what we must” to prevent Tehran from acquiring nuclear weapons.

Mr. Obama is set to address the United Nations General Assembly Tuesday. In excerpts of his speech released by the White House, the president tells Iran there is “still time and space” to negotiate over its nuclear program, but “that time is not unlimited.”

He also denounces the violence that spread across the Muslim world over a crudely made video mocking the Prophet Muhammad.

“There are no words that excuse the killing of innocents,” Mr. Obama says. “There is no video that justifies an attack on an embassy,” he adds, referring to the attack on the American consulate in Benghazi earlier this month that led to the deaths of U.S. Ambassador to Libya Christopher Stevens and three U.S. diplomatic officers.

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 in Syria “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 diffuse 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 may 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.