WASHINGTON (Reuters) - U.S. officials hope to prevent two diplomatic challenges with Iran from colliding next week: the Iranian nuclear program and the threat posed by the Islamic State militant group.

Their basic dilemma is how to keep Iran from hardening its stance in the nuclear talks out of a belief, which U.S. officials say would be misguided, that Washington might make nuclear concessions in exchange for help against IS.

A related problem is how to address the threat from the Sunni militant group without somehow enlisting the help of Iran, which has extensive influence in Iraq and in neighboring Syria.

A third difficulty is whether major Sunni states such as Saudi Arabia will be willing to participate in any coalition against the Islamic State group if Shi'ite Iran played any role.

"In the long run it will be difficult to find solutions without Iran," a senior Western diplomat said on condition of anonymity. "Mainly the Americans are against that, but there are still some members in the Arab region who are ... not very comfortable sitting with Iran at the same table," he added.

KERRY: IRAN HAS A ROLE

While the United States has repeatedly ruled out military "coordination" with Iran against IS, U.S. Secretary of State John Kerry said on Friday during a U.N. Security Council session on Iraq that he believed Tehran could play some role.

"The coalition required to eliminate ISIL is not only, or even primarily, military in nature," Kerry said, using the U.S. government's acronym for IS.

"It must be comprehensive and include close collaboration across multiple lines of effort," he said. "There is a role for nearly every country in the world to play, including Iran."

Whether Iran would play a role, without seeking a nuclear tradeoff, remains an open question.

Despite pessimism among diplomats on all sides, the United States hopes to make headway next week toward a deal to curb Iran's nuclear program in talks already under way in New York between Iran and major powers ahead of a Nov. 24 deadline.

Current and former U.S. officials say the United States can keep the nuclear talks separate from the effort to counter IS, though they acknowledge it is a concern for Washington.

"The Iranians could conceivably develop misconceptions in that regard but I think the administration would be very careful to try to avoid any misimpression and to make clear these are separate issues and that the U.S. isn't considering tradeoffs between them," said Robert Einhorn, a key U.S. official in talks with Iran until he left the State Department last year.

Suzanne Maloney, another former State Department official, now at the Brookings Institution think tank, said Iran might be emboldened to take a harder line in the nuclear negotiations simply because of the regional instability.

"I worry that the regional chaos is facilitating the Iranians' perception ... that playing for time is a reasonable strategy," she said, suggesting Iran might try to roll over an interim deal, the Joint Plan of Action, which gave it limited sanctions relief in exchange for curbing its nuclear program.

Maloney said the Iranians may conclude that "the U.S. is so overwhelmed with other priorities and has so few foreign policy victories to its credit at this stage that the president, in particular, might be persuaded to find a way if not extend the negotiations then at least extend the JPOA," she added.

CONVERGING INTERESTS?

On the sidelines of the nuclear talks between Iran and six major powers this week, U.S. officials had their latest in a series of bilateral discussions with Iran that focused on the nuclear issue but touched on the question of IS threat.

While they are long-time antagonists, U.S. and Iranian interests now appear to intersect in Iraq, where neither wishes to see the Shi'ite-led government lose more territory to IS.

U.S. officials fear the largely ungoverned territory IS has seized in Iraq and Syria could become a breeding ground for militants who then attack Western Europe and the United States.

They also do not wish to see Iraq totally unravel, which could endanger global oil supplies and call into question the high cost in money and American lives the United States expended in its attempt to stabilize the country after toppling Saddam Hussein.

For Iran, the extension of IS control in Iraq would undercut Iranian influence in its Shi'ite majority neighbor.

As a result, the two countries may find that despite their mutual distrust and the hostility of conservatives on both sides to their cooperation, they have some reason to make common cause against IS and could perhaps find a way to finesse the issue.

"You don't want to be asking the Iranians a lot for fear that they would then ask for something in return on the nuclear negotiations," said Jon Alterman, director of the Middle East program at the Center for Strategic and International Studies think tank in Washington. "But you can still talk about what we do and not do, and the Iranians can draw their own conclusions."

(Additional reporting by Michelle Nichols at the United Nations; Editing by Howard Goller)