UNITED NATIONS — President Barack Obama warned Iran on Tuesday the United States will “do what we must” to prevent it from acquiring a nuclear weapon and said time was running short for diplomacy but ignored the Israeli prime minister’s demand to set a “red line” for Tehran.

Taking the podium at the United Nations six weeks before the presidential election, Obama sought to counter criticism of his foreign record by Republican rival Mitt Romney, who has accused him of mishandling the Arab Spring uprisings, damaging ties with Israel and not being tough enough on Iran.

Obama also challenged world leaders to stand united against anti-American violence that has swept many Muslim countries in recent weeks and to promote tolerance amid anger over a crudely made video that offended Islam.

Seeking to step up pressure on Iran, Obama told the U.N. General Assembly that there was still scope for reaching a diplomatic solution but that “time is not unlimited.”

His tough talk appeared aimed at easing Israeli Prime Minister Benjamin Netanyahu’s concerns about U.S. resolve to curb Tehran’s nuclear drive, as he reasserted before the world body that he would never let Iran develop an atomic bomb and then simply contain the problem.

But even though Obama brought his warning for Iran to the forefront of the international stage this time, it was unclear whether that would be enough to placate Netanyahu, who has had a notoriously testy relationship with the President.

Obama stopped short of setting a specific “redline” that Iran must not cross if it is to avoid military action — something Netanyahu has openly demanded for weeks. Other than a few new rhetorical touches, the president did not appear to go further than he has in the past.

“A nuclear-armed Iran is not a challenge that can be contained,” he said. “It would threaten the elimination of Israel, the security of Gulf nations and the stability of the global economy.”

“The United States will do what we must to prevent Iran from obtaining a nuclear weapon.”

The unusually public dispute between the United States and Israel — the worst rift in decades — has been exacerbated by Obama’s decision not to meet with Netanyahu on his U.S. visit later this week, a move that risks alienating some pro-Israel voters.

Romney has attempted to exploit the situation politically, accusing Obama in the past of “throwing Israel under the bus.”

Though both governments have sought to calm the situation in recent days, Obama signaled resentment at Netanyahu’s tactics when, in an interview on CBS’s “60 Minutes” program broadcast on Sunday, he said he would ignore “noise that’s out there.”

There was no immediate reaction to Obama’s comments from Israeli leaders, who maintained near-total total silence as the country closed down for the holiest Jewish day of the year, Yom Kippur.

Netanyahu, who will address the General Assembly on Thursday, has indicated impatience over Obama’s entreaties to hold off on attacking Iran’s nuclear sites to give sanctions and diplomacy more time to work.