The United States tripled its estimate for the number of Islamic State fighters as it geared up for a "more aggressive" air campaign and Arab nations rallied behind President Barack Obama's nascent coalition against the jihadists.

French President Francois Hollande was on his way Friday to Baghdad where he will meet his Iraqi counterpart Fuad Masum in a show of support.

France has said it is prepared to participate in air strikes against the militants in Iraq "if necessary", and hosts an international conference on Iraq on September 15.

Ten Arab states including Saudi Arabia "agreed to do their share in the comprehensive fight" against Islamic State, said a statement after a meeting Thursday between US Secretary of State John Kerry and his Arab counterparts.

The militants now have about 20,000 to 31,500 fighters on the ground in Iraq and Syria, the Central Intelligence Agency said, much higher than a previous estimate of 10,000.

"This new total reflects an increase in members because of stronger recruitment since June following battlefield successes and the declaration of a caliphate, greater battlefield activity, and additional intelligence," CIA spokesman Ryan Trapani said.

The vastly higher estimate underscored the scale of the challenge after Obama vowed to expand an offensive against the extremists, a plan which foresees new air strikes against IS in Syria, expanded attacks in Iraq and new support for Iraqi government forces.

Iraq's new unity government and the Syrian opposition welcomed Obama's plan against IS, which has seized a swathe of Iraq and Syria and committed horrifying atrocities. But Syrian President Bashar al-Assad's regime and powerful ally Russia condemned it.

"Any action of any kind without the consent of the Syrian government would be an attack on Syria," National Reconciliation Minister Ali Haidar said.

A Russian foreign ministry spokesman said unilateral action would be a "crude violation" of international law.

Kerry retorted that he was "really rather surprised that Russia would dare to assert any notion of international law after what has happened in Crimea and eastern Ukraine".

Doubts about Obama plan

Obama said he was sending another 475 military personnel to help train Iraqi forces, but stressed that the campaign would not be a repeat of the exhausting ground wars fought by US troops in the past decade in Iraq and Afghanistan.

Instead, Washington is looking to empower partners on the ground such as Iraqi and Kurdish forces, as well as Syrian rebels.

US combat aircraft will soon start flying out of a base in the Kurdish region of Iraq as part of the campaign, the Pentagon announced.

"The kind of support we're going to be giving to Iraqi forces will be more aggressive from the air," said Pentagon spokesman Rear Admiral John Kirby.

Obama called on Congress to swiftly authorise an operation to train and equip moderate fighters, but several House Republicans said after a caucus meeting Thursday that a quick vote on that was unlikely.

House Speaker John Boehner said they were worried that the broader strategy was insufficient.

"If our goal is to eliminate ISIL, there's a lot of doubt whether the plan that was outlined by the president... is enough to accomplish that mission," he said.

Broadening the coalition

Along with the Saudis, Bahrain, Kuwait, Oman, Qatar and the United Arab Emirates, as well as Egypt, Iraq, Jordan and Lebanon are Arab parties to the coalition agreement.

A final statement declared a "shared commitment to stand united against the threat posed by all terrorism, including the so-called Islamic State in Iraq and the Levant".

The fight will include "stopping the flow of foreign fighters through neighbouring countries, countering financing of ISIL and other violent extremists, repudiating their hateful ideology, ending impunity and bringing perpetrators to justice".

It will also include humanitarian relief, aiding reconstruction and rehabilitation of communities "brutalised by ISIL".

On Friday Kerry heads to Ankara after Turkey refused to allow its air bases to be used in the campaign or to take part in combat.

A US official in Jeddah said Turkey had its reasons for staying out of the coalition. IS militants hold 49 Turks hostage, including diplomats and children, abducted from the Turkish consulate in Mosul in Iraq in June.

Meanwhile Syria's opposition urged Washington to take action against Assad as well as jihadists.

The opposition National Coalition said it had "long called" for action against IS and "warned time and again of the growing threat of this extremist group".

The US announcement was praised by Baghdad, where a unity government was formed Monday to address sectarian grievances that contributed to the rise of the jihadists.

IS has declared a "caliphate" in parts of Syria and Iraq it controls and is accused of committing savage murders, crucifixions and rapes, as well as selling women into slavery.

Two captive US journalists, James Foley and Steven Sotloff, were beheaded by the jihadists in recent weeks.