US warplanes carried out their first air strike on the Islamic State near Baghdad, as world diplomats pledged Monday to support Iraq in its fight against the militants.

The United States early last month began air strikes against IS positions in northern Iraq, but Monday's announcement that the campaign had targeted the jihadists near the Iraqi capital marks an escalation in the scope of the mission.

It comes less than a week after US President Barack Obama, in a primetime television address to the nation, ordered a "relentless" war against the Islamic State, including air strikes in Syria and expanded operations in Iraq to "destroy" the marauding jihadist army.

"US military forces continued to attack ISIL (IS) terrorists in Iraq, employing attack and fighter aircraft to conduct two air strikes Sunday and Monday in support of Iraqi security forces near Sinjar and southwest of Baghdad," US Central Command said in a statement.

"The air strike southwest of Baghdad was the first strike taken as part of our expanded efforts beyond protecting our own people and humanitarian missions to hit ISIL targets as Iraqi forces go on offense, as outlined in the president's speech last Wednesday."

The strikes destroyed six IS vehicles near Sinjar and an IS fighting position southwest of Baghdad that had been firing on Iraqi forces.

They bring the number of US air strikes across Iraq to 162.

'Any means'

It came as representatives from about 30 countries and international organisations, including the United States, Russia and China, met in Paris to discuss the crisis triggered when IS overran large areas of Iraq and Syria, carrying out beheadings and forced religious conversions.

The savage beheading at the weekend of a Briton, the third Western hostage to be executed on camera, raised the stakes in the battle against the jihadists, who have declared a caliphate and have as many as 31,500 fighters, according to the CIA.

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.

The Paris 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.

US Secretary of State John Kerry, attending the talks, stressed again that "we're not going to coordinate with the Syrians."

However he added that Obama had made it clear that "he will hunt down ISIL (Islamic State) wherever they may be, and that includes Syria."

Another senior US official, speaking on condition of anonymity, warned that US forces will target Syrian anti-aircraft systems if they take aim at American planes conducting strikes inside Syria against Islamic State rebels.

'No time to lose'

On the ground in Iraq, 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 ongoing 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."

The Paris 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.

Ten Arab states including Saudi Arabia are among the countries backing the US-led 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."

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.