* Israeli leader seeks to maintain cooperation with U.S.
* Clocks tick on Tehran's nuclear projects, U.S. election (Edits, adds leaked Israeli report on Iranian economy)
By Jeffrey Heller
NEW YORK, Sept 27 (Reuters) - Israeli Prime Minister Benjamin Netanyahu will set out, in his speech at the United Nations on Thursday, an ultimatum for Iran to halt its disputed nuclear drive or risk coming under military attack, an Israeli official said.
Netanyahu faces the world body after U.S. President Barack Obama disappointed some Israelis, in his own address to the annual assembly, by not calling for a deadline to be imposed on Tehran - though he did say time for diplomacy "is not unlimited".
Israel sees a mortal threat in a nuclear-armed Iran and has long threatened to strike its arch-foe pre-emptively, agitating war-wary world powers as they pursue sanctions and negotiations.
Complicating Netanyahu's strategy have been his testy relations with Obama as a U.S. election looms, and the reluctance of many Israelis to trigger a conflict with Iran, which denies that it is seeking to develop nuclear weapons and has pledged wide-ranging retaliation if attacked.
"The prime minister will set a clear red line in his speech that will not contradict Obama's remarks. Obama said Iran won't have nuclear weapons. The prime minister will clarify the way in which Iran won't have nuclear arms," a senior Israeli official said en route to New York, without elaborating.
Though he has not previously detailed when Israel might be willing to go to war, Netanyahu has said Iran could have enough low-enriched uranium by early 2013 to refine to a high level of fissile purity for a first nuclear device.
Israel worries that this final step, if taken, could happen too quickly or quietly to be prevented.
Iran has said it has no plans to enrich uranium beyond the 20 percent purity required to run a reactor producing medical isotopes. That level, however, brings raw uranium exponentially closer to the 90 percent enrichment required for bomb fuel.
Though reputed to have the Middle East's sole nuclear arsenal, Israel would be hard-put to deliver lasting damage to Iran's remote facilities using its conventional forces, or to handle a multi-front war.
Netanyahu's public calls for a U.S. ultimatum have deepened acrimony with Obama, a Democrat accused by his Republican rivals of being soft on the Jewish state's security. That has stirred American accusations of Israeli meddling in the November presidential elections - something denied by Netanyahu.
"The prime minister will say that Israel and the United States can work together to achieve their common goal," said the Israeli official on condition of anonymity.
Netanyahu, who heads a broad-based, conservative coalition government, departed for New York on Wednesday saying he would take the U.N. podium for an Israel "united in the goal of preventing Iran from achieving nuclear weaponry".
But surveys show that most Israelis - apparently swayed by the open dissent of several senior national-security figures - would oppose launching unilateral strikes on Iran, given the risk of alienating Washington and of provoking clashes with Tehran's Islamist militant allies in Lebanon and Gaza.
A poll published by the liberal Haaretz newspaper on Thursday found that 50 percent of Israelis feared for the survival of their country, should there be a conflict.
Iranian President Mahmoud Ahmadinejad, in his speech to the General Assembly on Wednesday, said Iran was under threat of military action from "uncivilized Zionists," a clear reference to Israel. Earlier this week, Ahmadinejad said that Israel would eventually be "eliminated."
Haaretz also ran excerpts from a leaked Foreign Ministry report that sanctions had caused greater damage to Iran's economy than anticipated by Israel.
The findings, confirmed to Reuters by an Israeli official, could undermine any attempt by Netanyahu to argue that the military alternative must be considered imminently.
Israeli opposition leader Shaul Mofaz criticised Netanyahu for sparring with Obama and voiced confidence in U.S. resolve.
"I am convinced that the United States, the president of the United States, is determined to prevent Iran going nuclear," Mofaz told Israel's Army Radio.
Even within Netanyahu's coalition there have been misgivings about the pitch of disagreement with the United States.
Danny Ayalon, deputy to Foreign Minister Avigdor Lieberman, described Obama's Iran remarks at the United Nations as "important, albeit measured".
Speaking on Israel Radio, Ayalon said the Netanyahu government and Obama administration were in discreet contacts and approaching agreement on setting limits for Iran.
(Writing by Dan Williams; Editing by Angus MacSwan)