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 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 Friday in support of the so-called '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 Agence France Presse.

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 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.

The United States has trained and equipped a number of fighters - screened and determined to be "moderate" - to operate against the so-called 'Islamic State of Iraq and the Levant' (ISIL) takfiri organization.