French fighter jets took off from the UAE on Monday as France joined Britain in conducting spy flights over Iraq in support of US air strikes against Islamic State militants.

"… the first reconnaissance flights will be carried out in agreement with the Iraqi and Emirati authorities," AFP quoted French Defence Minister Jean-Yves Le Drian as saying to French troops, including pilots, at the Al-Dhafra base in the United Arab Emirates shortly before two Rafale jets took off from the base.

Britain had already been conducting surveillance flights over Iraq in support of the US air strikes but has so far carried no air strikes on Islamic State targets.

Le Drian's visit came as a one-day international conference on Iraq opened in Paris to discuss political, security and humanitarian aspects of united international action to tackle the threat from Islamist jihadists, who already control  quarter of land in Syria and 40% of Iraq and could number as many as 31,500 fighters, according to CIA estimates.

The conference has been given added urgency by the beheading of British aid worker David Haines on Saturday. Haines was the third Western hostage to be beheaded by the militants in less than a month.

"France stands ready at a time that is decisive for its security — because it is also France's security that is threatened by this... pseudo-Islamic State," Le Drian said.

The United States has unveiled a plan to fight the jihadist militants simultaneously in Iraq and Syria. It believes it can forge a solid alliance despite hesitancy among some partners and questions over the legality of action.

Highlighting that the Islamic State group poses a threat to global security, French President Francois Hollande said at the opening of the conference that “… the response must be global ... Iraq's fight against the terrorists is also our fight. We must commit ourselves together - that is the purpose of this conference."

According to a US State Department official, while no Arab nations have publicly committed to military action, several have told the US privately they are willing to join in airstrikes in Iraq and in Syria, Bloomberg reported.

So far, the US is the only foreign nation carrying out airstrikes against the jihadist group in Iraq. During Hollande’s visit to Baghdad last week, Iraqi Prime Minister Haidar al-Abadi said he was told that France “will take part in striking terror locations.”