Washington (AFP) - 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."