By Matt Spetalnick and Mark Felsenthal
NEW YORK (Reuters) - President Barack Obama will warn Iran on Tuesday that the United States will "do what we must" to prevent it acquiring a nuclear weapon, and appeal to world leaders for a united front against further attacks on U.S. diplomatic missions in Muslim countries.
Preparing to take the podium at the United Nations six weeks before the U.S. presidential election, Obama hopes 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.
Seeking to step up pressure on Iran, Obama will tell the U.N. General Assembly that there is still time for a diplomacy but that "time is not unlimited," according to advance excerpts of his speech, due to begin sometime around 1315 GMT.
His tough talk appears aimed at easing Israeli concerns about U.S. resolve to curb Tehran's nuclear drive, as he reasserts before the world body that he will never let Iran develop an atomic bomb and then simply contain the problem.
But he will stop short of meeting Israeli Prime Minister Benjamin Netanyahu's demand to set a clear "red line" that Iran must not cross if it is to avoid military action.
"A nuclear-armed Iran is not a challenge that can be contained," he will say. "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."
Obama is also seeking to reassure voters that he is doing everything he can to head off any more violence like the September 11 attack in Libya that killed the U.S. ambassador and three of his colleagues.
A wave of Muslim anger over a crude anti-Islam video made in California has swept the Middle East, North Africa and South Asia. Americans were stunned by images of U.S. flags again burning in the Muslim world, the focus of intense personal diplomacy by the president at the start of his term.
"The attacks of the last two weeks are not simply an assault on America. They are also an assault on the very ideals upon which the United Nations was founded," Obama will tell the assembled world leaders.
This eruption of violence has confronted Obama with the worst setback yet in his efforts to keep the Arab Spring revolutions from turning against the United States - and has demonstrated that he has few easy options.
In his speech, he has the delicate task of articulating U.S. distaste for insults to any religion while at the same time insisting there is no excuse for a violent reaction - a distinction rejected by many Muslims.
The crisis has exposed a deep divide over the issues of free speech and blasphemy at a time when Islamist forces are in the political ascendant in the Middle East after several veteran dictators were ousted.
"Today, we must declare that this violence and intolerance has no place among our United Nations," Obama will say.
Obama is not expected to offer detailed solutions to an array of crises that threaten to chip away at a foreign policy record that his aides hoped would be immune from Republican attack.
ELECTION THE PRIORITY
With campaign pressures building in a close race, Obama's final turn on the world stage before facing voters has left little doubt about his immediate priorities.
He skipped the customary one-on-one meetings with foreign counterparts but went ahead with the taping of a campaign-style appearance on the popular television talk-show "The View" - a trade-off that drew Republican criticism.
Obama planned to be in and out of New York in 24 hours and off to the election battleground state of Ohio on Wednesday.
Despite Obama's international woes, administration officials are heartened by Romney's own recent foreign policy stumbles and doubt that the president's critics will gain traction in a campaign that remains focused mainly on the U.S. economy.
In addition, the White House never tires of touting the killing of Osama bin Laden and the ending of the Iraq war as Obama's foreign policy accomplishments.
Nevertheless, the unsettled climate surrounding Obama's U.N. visit will be a stark reminder that the heady optimism that greeted him when he took office, and was awarded the Nobel Peace Prize within months, has now cooled.
Obama's early overtures to Iran were rejected, and the expansion of Tehran's nuclear program, which it says is purely peaceful, has created tension between Washington and Israel, which sees a nuclear-armed Iran as a threat to its existence.
Netanyahu 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.
Underscoring the depth of the problem, Iranian President Mahmoud Ahmadinejad said in New York on Monday that Israel had no roots in the Middle East and would be "eliminated."
The White House dismissed his comments as "disgusting."
Despite that, the unusually public dispute between the United States and Israel 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.
Signaling resentment at Netanyahu's tactics, Obama told CBS's "60 Minutes" he would ignore "noise that's out there."
(Editing by Kevin Liffey)