Iran on Monday spurned an American request for cooperation in the fight against Islamic State militants, but the United States said the door remains open to a rare opportunity to make common cause with its principal adversary in the Middle East.

Iran’s rebuff came as world powers meeting in the French capital agreed to use “any means necessary” to combat the militant force surging in Iraq and Syria.

Diplomats from 26 nations and several international organizations began dividing responsibilities for what U.S. Secretary of State John F. Kerry said will be an expanded international military, diplomatic and law enforcement assault on the group.

“It’s not the Iraq war of 2003,” Kerry told reporters Monday. “We’re not building a military coalition for an invasion. We’re building a military coalition, together with all the other pieces, for a transformation.”

The sudden rise of the Islamic State has not only rearranged old rivalries and alliances but also eclipsed Syria’s civil war and Iraq’s sectarian fragmentation as the most pressing threat in the Middle East.

Secretary of State John Kerry, right, shakes hands with Russian Foreign Minister Sergey Lavrov at an international conference in Paris to determine a response to the growing threat posed by the Islamic State. (Brendan Smialowski/AP)

The notion that the United States might find its concerns shared by foes Iran and Syrian President Bashar al-Assad is startling, as is the emerging partnership between Saudi Arabia, the spiritual center of Sunni Islam, and Shiite-led Iraq.

Kerry noted Monday that the support building for Iraq as it confronts the militants would have been unthinkable only a few months ago.

As the Paris talks opened — without representatives from Iran or Syria — French fighter jets flew a reconnaissance mission over Iraq.

France is alone in publicly offering to join the United States in flying bombing missions against Islamic State targets in Iraq, but Arab states have signaled willingness behind the scenes.

The goal is to back up Iraqi ground forces trying to reverse the militant gains in western and northern Iraq. A statement from the diplomats in Paris made no mention of Syria, where the militants have carved out a haven in the midst of the country’s war.

In a show of support for Iraq’s new leadership, the conference participants pledged to expunge the militants from territory seized in Iraq “by any means necessary, including appropriate military assistance.”

Iran played spoiler. Its supreme leader, Ayatollah Ali Khamenei, tweeted his disdain for the international effort and revealed a back-channel U.S. offer of unspecified cooperation against the militants. Khamenei said Iran rejected the U.S. request because of Washington’s “evil intentions,” the state-run Islamic Republic News Agency reported.

The Reuters news agency quoted Khamenei as telling Iran’s state television that the U.S. request was “hollow and self-serving,” echoing Iran’s claims that Western nations are seeking to expand their influence in the region as part of the campaign against the Islamic State.

The United States did not deny the outreach to Iran and said discussions with Tehran will continue — underscoring Iran’s influence in the region as well as the political complexities of bringing the Shiite powerhouse into the emerging international alliance against the Islamic State.

“I’m just going to hold open the possibility always of having a discussion that had the possibility of being constructive,” Kerry said, without providing substantive details about the U.S. request. “I’m not going to get into a back-and-forth.”

By going public with the U.S. offer Monday, Iran appeared to close off the possibility of cooperation against the militants for now.

However, Iran has sent its allied Shiite militias in Iraq to fight with Western-backed Kurds against the Islamic State. Iran’s Shiite theocracy considers the Sunni militants a challenge to Iraq’s majority Shiites — whose political parties have close ties to Tehran — and a destabilizing force against Assad, Iran’s other main regional ally.

Although details of the U.S.-Iranian discussion remain vague, it appears to have been an offer of behind-the-scenes cooperation rather than a public partnership.

Any public cooperation with Iran would doom the emerging alliance between Iraq and Sunni Arab states in the Persian Gulf region and elsewhere that had feuded with Nouri al-Maliki, the former Iraqi prime minister. The Sunni states regard Iran with deep suspicion and considered Maliki, a Shiite partisan with strong ties to Tehran, as a pawn of Iran.

France had wanted to invite Iran to the talks, but the United States resisted the move.

The United States is trying to stitch together a diverse alliance against the Islamic State and overcome reluctance among many states to intervene in any way in the Syrian conflict, now in its fourth year. Nearly 200,000 people have died in the Syrian fighting, according to the United Nations.

Kerry said Monday that Saudi King Abdullah had told him that if Iran attended Monday’s session, the Saudis would boycott. The United Arab Emirates had drawn the same line, Kerry said.

As the international efforts gathered steam, the leader of a key Iran-linked militia in Iraq pledged Monday to pull back from any area where U.S. forces intervene, including with possible aerial attacks.

Shiite cleric Moqtada al-Sadr said his forces would pull out of any area where the United States launches attacks, “whether by land or sea, directly or indirectly. His forces waged fierce battles against U.S. troops in the years after the American-led 2003 invasion.

Opening the Paris conference, French President François Hollande said the threat from global militancy requires a coordinated and international response. France is among the European nations deeply alarmed by the flow of radicalized young men who have traveled from Europe to fight in Syria and who could seek to return home.

The meeting came at the end of Kerry’s week-long tour of Iraq, Saudi Arabia, Egypt and Turkey. The trip sought to frame the division of labor for a wider assault on the Islamic State, with the U.S. military and Iraqi forces playing the central roles.

On Sunday, U.S. officials said Arab states have volunteered to launch airstrikes alongside U.S. planes. But they stressed that such an expansion was still under discussion and subject to review by Iraq.

Officials from the region said the volunteers included Saudi Arabia, the United Arab Emirates and others whose leaders had been waiting to hear from the administration that it has a viable plan and is prepared to follow through with it.

The United Arab Emirates and Qatar conducted strikes during the 2011 air campaign in Libya. Qatar’s role is not entirely clear now, though it is helping train Syrian rebels, as is Jordan.

Saudi Arabia is also expected to participate in expanded training of the rebels fighting the Islamic State and Assad. The Saudis have been pressing the United States to accede to Syrian rebels’ long-standing requests for surface-to-air antiaircraft weapons, which could be a game-changer for the chronically underequipped opposition forces, but the Obama administration has refused.

The U.S. decision to confront the militants, first in Iraq and eventually in Syria, also benefits Assad, although U.S. officials insist they will act only in their own interests.

Kerry ruled out coordination with Syria.

Brian Murphy in Washington and Loveday Morris in Baghdad contributed to this report.