PARIS/DUBAI (Reuters) - Iran's supreme leader said on Monday he had personally rejected an offer from the United States for talks to fight ISIS, an apparent blow to Washington's efforts to build a military coalition to fight militants in both Iraq and Syria.

World powers meeting in Paris on Monday gave public backing to military action to fight ISIS (Islamic State in Iraq and Syria) fighters in Iraq. France sent jets on a reconnaissance mission to Iraq, a step towards becoming the first ally to join the U.S.-led air campaign there. But Iran, the principal ally of Islamic State's main foes in both Iraq and Syria, was not invited to the Paris meeting.

The countries that did attend - while supporting action in Iraq - made no mention at all of Syria, where U.S. diplomats face a far tougher task building an alliance for action.

Washington has been trying to build a coalition to fight Islamic State since last week when President Barack Obama pledged to destroy the militant group on both sides of the Iraqi-Syrian border. That means plunging into two civil wars in which nearly every country in the Middle East already has a stake. And it also puts Washington on the same side as Teheran, its bitter enemy since the Islamic revolution of 1979.

In a rare direct intervention into diplomacy, Iran's supreme leader, Ayatollah Ali Khamenei, said Washington had reached out to Iran through the Iranian embassy in Baghdad, requesting a meeting to discuss cooperation against ISIS. Khamenei said that some Iranian officials had welcomed the contacts, but he had personally vetoed them.

"I saw no point in cooperating with a country whose hands are dirty and intentions murky," the Iranian leader said in quotes carried on state news agency IRNA. He accused Washington of "lying" by saying it had excluded Iran from its coalition, when in fact Iran had refused to participate.

U.S. Secretary of State John Kerry said Washington was "not cooperating with Iran", but declined to be drawn on whether it had reached out through the embassy in Baghdad for talks.

"I am not going to get into a back and forth," he said. "I don't think that's constructive, frankly."

ISIS fighters set off alarms across the Middle East since June when they swept across northern Iraq, seizing cities, slaughtering prisoners, proclaiming a caliphate to rule over all Muslims and ordering non-Sunnis to convert or die.

ISIS fighters, known for beheading their enemies or captives, raised the stakes for the West by cutting off the heads of two Americans and a Briton in videos posted on the Internet which showed the prisoners bound in orange jumpsuits.

French officials said they had hoped to invite Iran to Monday's conference but Arab countries had blocked the move. "We wanted a consensus among countries over Iran's attendance, but in the end it was more important to have certain Arab states than Iran," a French diplomat said.

Iran sponsors the governments of both Iraq and Syria and has been at the centre of defences against ISIS in both countries. The United States reached out to Iran last year when secret talks led to a preliminary deal on nuclear issues.

Iran has occasionally played down its conflicts with the West since President Hassan Rouhani, a relative moderate, was elected last year. Khamenei's intervention, including his statement that some Iranian officials welcomed the U.S. overture, was a rare public acknowledgment of division but also a reminder that powerful interests in Iran oppose a wider thaw.

At Monday's international conference in Paris, the five U.N. Security Council permanent members, European and Arab states and representatives of the EU, Arab League and United Nations all pledged to help the Baghdad government fight ISIS.

"All participants underscored the urgent need to remove Daesh from the regions in which it has established itself in Iraq," said a statement after the talks. Daesh is an Arabic acronym for ISIS which now calls itself the Islamic State.

"To that end, they committed to supporting the new Iraqi Government in its fight against Daesh, by any means necessary, including appropriate military assistance...." it said.

Foreign Minister Laurent Fabius said French aircraft would begin reconnaissance flights over Iraq. A French official said two Rafale fighters and a refuelling aircraft had set off.

"The throat-slitters of Daesh - that's what I'm calling them - tell the whole world 'Either you're with us or we kill you'. And when one is faced with such a group there is no other attitude than to defend yourself," Fabius told a news conference at the end of the talks.

Iraqi President Fouad Massoum told Monday's conference he hoped the Paris meeting would bring a "quick response".

"Islamic State's doctrine is either you support us or kill us. It has committed massacres and genocidal crimes and ethnic purification," he told delegates.

Monday's conference was an important vote of confidence for the new Iraqi government, formed last week, led by a member of Iraq's Shi'ite majority, Prime Minister Haider al-Abadi, and also including minority Sunnis and Kurds in important jobs.

Iraq's allies hope Abadi will prove a more consensual leader than his predecessor Nuri al-Maliki, a Shi'ite whose policies alienated many Sunnis, and that the new government will win back support from Sunnis who had backed the ISIS revolt. The broad international goodwill towards Abadi shown at Monday's conference means Washington will probably face little diplomatic pushback over plans for air strikes in Iraq.

Syria, however, is a much trickier case. In a three-year civil war, ISIS has emerged as one of the most powerful Sunni groups battling against the government of President Bashar al-Assad, a member of a Shi'ite-derived sect.

Washington and its allies remain hostile to Assad, which means any bombing is likely to take place without permission of the Damascus government. Russia, which backs Assad, says bombing would be illegal without a resolution at the U.N. Security Council, where it has a veto.

Meanwhile, Turkey and other enemies of Assad are wary of measures against ISIS that might help him. The United States resumed air strikes in Iraq in August for the first time since the 2011 withdrawal of U.S. troops.

Obama's plans, announced last week, involve stronger military action in Iraq and extending the campaign to Syria. Kerry has said he believes he can forge a solid alliance despite hesitancy among some partners and questions over the legality of action.

U.S. officials said several Arab countries have offered to join the United States in air strikes against ISIS targets, but declined to say which countries made the offers.

Ten Arab states committed last week to joining a military coalition, without specifying what action they would take.

Britain, Washington's main ally when it invaded Iraq in 2003, has yet to confirm it will take part in air strikes, despite the killing of British aid worker David Haines by ISIS fighters this past week.

France has said it is ready to take part in bombing missions in Iraq but is so far wary of action in Syria. French officials say the coalition plan must go beyond military and humanitarian action, arguing there must also be a political plan for once ISIS has been weakened in Iraq.