The world's top diplomats pledged Monday to support Iraq in its fight against Islamic State militants by "any means necessary", including "appropriate military assistance", as leaders stressed the urgency of the crisis.

Representatives from around 30 countries and international organisations, including the United States, Russia and China, gathered in Paris as the savage beheading at the weekend of a third Western hostage raised the stakes in the battle against the marauding jihadists.

In a joint statement, diplomats vowed to support Baghdad "by any means necessary, including appropriate military assistance, in line with the needs expressed by the Iraqi authorities, in accordance with international law and without jeopardising civilian security."

They stressed that IS extremists were "a threat not only to Iraq but also to the entire international community" and underscored the "urgent need" to remove them from Iraq, where they control some 40 percent of the territory.

However, the statement made no mention of Syria, where extremists hold a quarter of the country and where Bashar al-Assad's regime still had friends around the Paris conference table, including Russia.

Opening the conference, French President Francois Hollande said there was "no time to lose" in the fight against the jihadists.

"The fight of the Iraqis against terrorism is our fight as well," Hollande said, urging "clear, loyal and strong" global support for Baghdad.

Co-hosting the meeting, Iraqi President Fuad Masum warned that the militants could overrun more countries in the region.

"We are still asking for regular aerial operations against terrorist sites. We have to pursue them wherever they are. We need to dry up their sources of finance," the Iraqi leader said.

The international community is scrambling to contain the IS jihadists -- who have rampaged across Iraq and Syria and could number as many as 31,500 fighters, according to the CIA.

In Iraq on Monday, sporadic clashes broke out near the town of Dhuluiyah, north of Baghdad, where security forces and allied tribesmen prepared for an operation against IS-led militants.

The area would appear to be the target of the next major drive against IS in Iraq, after a successful operation to break the siege of the town of Amerli farther north.

As if to stress the urgency of the campaign against IS, France's defence minister announced just hours ahead of the conference that Paris was joining Britain in carrying out reconnaissance flights in support of the US air campaign.

Shortly afterwards, two French Rafale fighter jets took off from the Al-Dhafra base in the United Arab Emirates.

And in Brussels, NATO chief Anders Fogh Rasmussen urged military action, calling IS "a group of terrorists with whom there is no chance whatsoever to negotiate."

- All bases covered -

The meeting was the latest in a series of frantic diplomatic efforts to build a broad global coalition against the jihadists, and German Foreign Minister Frank-Walter Steinmeier said meetings would come "thick and fast" ahead of a UN general assembly next week.

US Secretary of State John Kerry, who has been criss-crossing the region to drum up support, said at the weekend that "all bases were covered" in terms of implementing the US strategy to destroy the jihadists.

Ten Arab states including Saudi Arabia are among the countries backing the coalition, and Australia has pledged 600 troops.

"We are not building a military coalition for an invasion... but for a transformation as well as for the elimination of ISIL," Kerry told reporters, using an alternative name for IS.

"We are fighting an ideology, not a regime."

While there was no mention of Syria in the final statement, Hollande said the international community "needs to find a durable solution in the place where the (IS) movement was born. In Syria."

Hollande said the moderate Syrian opposition should be "backed by all means".

And Britain's Foreign Secretary Philip Hammond said that while strikes on Syria would be a far more complicated matter than in Iraq, "we haven't ruled it out."

- 'Dirty hands' -

However, Iran, which was not invited to the conference, said it had rejected US overtures to help in the fight against the militants.

Iran, like Iraq, is majority Shiite, while IS is made up of Sunni fighters who target Shiite Muslims.

"Right from the start, the United States asked through its ambassador in Iraq whether we could cooperate," supreme leader Ayatollah Ali Khamenei said on his official website.

"I said no, because they have dirty hands," said Khamenei, who has the final say on all matters of state in Iran.

Kerry ruled out cooperating with Iran on military action but said he was "open to have a conversation" with Tehran over Iraq.

Hammond also struck a conciliatory note saying: "We should continue to hope that Iran will align itself broadly with the direction that the coalition is going."

Iraq's Foreign Minister Ibrahim al-Jaafari voiced "regret" Iran had not been invited to the conference.

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