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The United States on Monday said it has used air power in Syria in defense of allied rebel groups, signaling deeper involvement in the country's brutal four-year civil war.

The Pentagon confirmed that an air strike was carried out on Friday in support of the New Syria Force, a US-allied group.

"We'll take action to defend the New Syria Force that we've trained and equipped," Pentagon spokesman Commander Bill Urban told AFP. 

He said "last Friday was the first one," referring to the air strike.

Earlier, a senior administration official said the United States had hit al-Qaeda's Syrian affiliate the al-Nusra Front in response to attack on US trained rebels.

President Barack Obama's administration said on Monday it was prepared to take "additional steps" to defend US-trained and equipped forces, warning Bashar al-Assad's regime "not to interfere."

"The president approved this recently upon the recommendation of his senior military advisers," a senior administration official told AFP.

White House spokesman Josh Earnest said Assad's regime had not so far hampered US-backed forces, but he nonetheless raised the possibility of strikes against it should the need arise.

The United States, Earnest said, was "committed to using military force where necessary to protect the coalition-trained and equipped Syrian opposition fighters."

The decision was taken under a 2001 rule authorizing the use of military force against terror groups, which critics say has already been stretched too far.

Officials argue that authority includes the ability to provide "defensive fire support."

The United States has trained and equipped a number of fighters - screened and determined to be "moderate" - to operate against the militant Islamic State group (IS, formerly ISIS).

But US-backed forces have yet to play a major role in turning the war and its fledgling local ground force has already suffered a series of reversals.

The al-Nusra Front seized at least five rebels from Division 30 between Monday and Tuesday, according to the Syrian Observatory for Human Rights, and had previously attacked the same force, which has a 54-strong unit from the US-trained rebels, with several members reportedly killed or captured.

Micah Zenko of the Council on Foreign Relations said the "truly significant decision" could potentially extend well beyond that small force.

US forces are "interspersed among large coherent units of several hundred fighters," he said, explaining that: "You can't give air cover just to individual rebels."

Turkey cooperation

The United States recently agreed with Turkey to create what has been termed an "Islamic State-free zone" in northern Syria.

Details of the zone "remain to be worked out", according to a senior administration official, who asked not to be named.

It would, however, entail Turkey, NATO'S only mainly Muslim member, supporting US "partners on the ground" already fighting the militants.

Ankara has also granted the United States permission to use one of its bases to carry out air raids against the group.

Washington has long pushed for the use of the Incirlik base due to its location relatively close to Syria just outside the Turkish city of Adana, but Turkey had hesitated for months.

Monday's announcement comes as diplomatic efforts to halt the carnage in Syria resume.

An estimated 140,000 people have died in the conflict, which began as an uprising against the Assad regime but has morphed into a multipronged religious and ethnic civil war.

A UN envoy recently presented his plan to resuscitate failed talks and foreign ministers from the United States, Russia and Saudi Arabia held talks in Qatar on Monday.

The trio agreed to the "need for a meaningful political transition" according to State Department spokesman Mark Toner.

But Russian Foreign Minister Sergei Lavrov later condemned Washington's move toward a more robust involvement in Syria.

"We believe it's counterproductive to announce publicly that some US-trained armed groups... will be under the protection of the coalition's air forces," Lavrov said.

Separately on Monday, the US Treasury Department imposed sanctions on seven entities and four individuals its says are providing energy products to Assad's regime, and named seven vessels as blocked property.