Thirty countries have pledged to help Iraq in the fight the al-Qaeda breakaway group ISIL "by any means necessary", as leaders gathered to discuss their strategy.

French president Francois Hollande called for a global response to counter ISIL on Monday, as he opened a conference on Iraq, bringing together members of a US-led coalition.

"[The threat] is global so the response must be global," the French leader said, at a Paris conference aimed at coordinating a strategy against the group, which controls parts of northern Iraq and Syria.

Foreign ministers from the main European states, the five permanent members of the UN Security Council, Iraq's neighbours and Gulf Arab states Qatar, Saudi Arabia, Kuwait, and the UAE, are in Paris to discuss broad political, security and humanitarian aspects of tackling ISIL.

In holding this conference, the countries meeting today are showing their solidarity and the will to protect themselves against terrorism," Hollande added in a joint news conference with Iraqi president Fouad Massoum.

Massoum said ISIL fighters were responsible for some of the worst atrocities committed in Iraq's history.

"We should spend more efforts, and therefore we ask to continue the air strikes against the terrorist positions. We will not give them any safe haven," Massoum added.

Al Jazeera's Nadim Baba, reporting from Paris, said that the statements released by the two leaders were not specific in terms of military action.

"What is really crucial is not who is going to carry out the air strikes but who will provide financial and military assistance."

The conference convened as Tehran claimed it had rejected a US request to cooperate against ISIL, citing Washington's "evil intentions", despite the US insisting it would not coordinate militarily with Iran.

"The United States asked through its ambassador in Iraq whether we could cooperate against [ISIL]... I said no, because they have dirty hands," said Iran's leading religious and political figure, Ayatollah Ali Khamenei.

French officials say the coalition against ISIL must go beyond military and humanitarian action, arguing there must also be a political plan for once ISIL has been weakened in Iraq.

They argue that the 2003 US-led intervention in Iraq, in which Paris did not participate, ultimately contributed to the current crisis.

France has said it is ready to join US air attacks in Iraq but says legal and military limitations make it more difficult in Syria, where ISIL's main power base lies.

French aircraft will begin reconnaissance flights over Iraq on Monday at the request of the Iraqi government.