With momentum building for international military action against Islamist extremists in Syria, the United States reiterated Friday it will not coordinate with strongman Bashar al-Assad in the effort.

"The answer to the security challenges is not the Assad regime," State Department deputy spokeswoman Marie Harf told reporters.

"They have created this security vacuum. We are not going to be working with them in this fight," she added. "They have lost legitimacy to lead, period."

An Assad advisor, Buthaina Shaaban, was quoted by state news agency SANA earlier Friday saying Damascus must be included as a "key part" of the multinational coalition that US President Barack Obama announced Wednesday in his strategy to defeat jihadists known as the Islamic State (IS).

Harf stressed Washington's firm anti-Assad line.

Government forces "continue to kill their own people," she said, referring to the devastating four-year civil war that has left more than 160,000 people dead and sparked a massive humanitarian crisis.

Asked whether US officials might need to coordinate with Damascus on issues such as ground troop movements, or in the event of a downed aircraft or captured US troops, Harf said "we do not believe that we do. We will not be coordinating with the Assad regime."

US lawmakers have spoken of the extraordinarily complex situation on the ground in Syria, where various rebel groups are fighting Assad forces.

Some of those, including moderate groups which the Obama administration wants to equip and train, stand accused of cooperating with extremists linked to Al-Qaeda and IS.

House Intelligence Committee member Peter King acknowledged that eradicating jihadists might empower the regime.

"There is definitely realization that in Syria, actions we take against ISIS could at least temporarily strengthen Assad, there's no doubt," King told reporters Thursday.

In addition, the State Department said the so-called Geneva process aimed at ending the civil war is unlikely to move forward until there is a political framework for transitioning Assad out of power.

"We will not have a third round until the regime makes clear it will come to the table ready to discuss that kind of transitional governing body," Harf said. "They have refused to do so."

Arabs back anti-IS move as Syria warns on 'attacks'
Jeddah, Saudi Arabia (AFP) Sept 11, 2014 - Arab nations rallied Thursday behind US President Barack Obama's call to expand operations against jihadists in Iraq and Syria, as Damascus warned it would consider any action on its territory as an attack.

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

Iraq's new unity government and the Syrian opposition welcomed Obama's plan, 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.

Obama said Wednesday he had ordered the US military to expand its operations against IS, which has seized a swathe of Iraq and Syria and committed horrifying atrocities.

"Our objective is clear: we will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy," Obama said, using an alternative acronym for the group.

"I will not hesitate to take action against ISIL in Syria, as well as Iraq."

He 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.

Instead, Washington is looking to empower partners on the ground like 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 said.

- Broadening the coalition -

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.

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

Participation will include "as appropriate, joining in the many aspects of a coordinated military campaign against ISIL".

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" and "supporting states that face the most acute ISIL threat".

Kerry told reporters that Arab partners will play a "leading role" in the coalition.

On Friday, he 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.

"Turkey remains an important partner on counterterrorism, and we will continue to consult closely as we work together to address the threat from ISIL," the official said.

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 grievances that contributed to the rise of the jihadists.

- 'Violation of international law' -

"Iraq welcomes Obama's strategy about standing with it in its war against (IS) and the terrorist groups," Prime Minister Haidar al-Abadi's office said.

But Russia said unilateral action would be a blatant violation of international law.

"In the absence of an appropriate decision of the UN Security Council, such a step would become an act of aggression, a crude violation of the norms of international law," a foreign ministry spokesman said.

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".

The growing IS threat was made clear after the group seized large parts of Iraq in a lightning June offensive, sweeping aside ineffective Iraqi forces.

It declared a "caliphate" in parts of Syria and Iraq it controls and has committed widespread atrocities, including beheadings, crucifixions, rapes and selling women into slavery.

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