Al-Qaeda’s Syrian affiliate threatened to attack all U.S.-backed rebels entering the country, after the U.S.-led coalition launched airstrikes on the jihadists’ positions in Syria in retaliation for the abduction of members of a rebel brigade trained by the Pentagon.

The airstrikes came as Jabhat al-Nusra, the al-Qaeda-linked group, announced the Thursday capture of Nadim al-Hassan, commander of the rebel group known as Division 30, which it said had received U.S. training and was acting on behalf of “agents of America” in Syria.

Syria’s Sunni Muslims “will not accept” groups working for the United States, the statement from Jabhat al-Nusra said. The group also called on the rebels to abandon the United States as an ally, and return to fighting in the name of Islam.

Division 30 includes about 60 Syrian fighters that the United States trained in recent months in Turkey and calls the New Syrian Force. They were sent into Syria two weeks ago to help build the moderate opposition to President Bashar al-Assad and participate in a new U.S.-Turkey effort to drive the Islamic State from northwest Syria.

U.S. officials have insisted that none of the newly trained fighters are among those abducted with Hassan, at a spot north of the Syrian city of Aleppo.

But they acknowledged that the captures of Hassan and the others were a blow to the Obama administration’s efforts. Jabhat al-Nusra is one of the most powerful Syrian insurgent groups but is not allied with the Islamic State.

Lt. Cmdr. Kyle Raines, spokesman for the U.S. Central Command, said that “members of the New Syrian Force, along with members of the 30th Division of the Free Syrian Army” had “successfully repelled” a new attack Friday morning by an “unknown force of about 50 personnel.”

“There was support from coalition airstrikes,” Raines said, but no confirmation of losses on either side. Nearly all the U.S. airstrikes on Syria that began in September have been directed against the Islamic State in central and eastern Syria.

The fights with Jabhat al-Nusra took place in the northwestern Syrian town of Azaz, near the Turkish border at Kilis, U.S. officials and a Syrian rights group said.

Syrian activists posted images of what they said were coalition strikes on a ­Jabhat al-Nusra command center in Azaz. In a statement posted online, Jabhat al-Nusra said the strikes “left a number of martyrs and wounded in our ranks.”

The statement also claimed Hassan’s capture and accused the U.S.-linked rebels of coordinating airstrikes on its positions in the area.

“We warn soldiers of [Division 30] against proceeding in the American project,” the statement said, according to Reuters.

A fighter who said he belonged to Jabhat al-Nusra posted an English-language account of Friday’s battle with Division 30 online.

The fighter, posting with the user name @Abu_M0hammed, said that the jihadists’ and U.S.-backed rebels clashed for about 30 minutes before they heard the sounds of a coalition drone overhead. The drone, @Abu_M0hammed said, was “collecting information” for later airstrikes on the group.

It taught “us some valuable lessons in how to improve our tactics on the ground, against enemies [who] have superior air power,” he said.

Pentagon officials said Thursday that none of the approximately 60 U.S.-trained fighters under Hassan’s command were among those captured by Jabhat al-Nusra.

Northwestern Syria has been a swirl of fighting groups, with other moderate, U.S.-backed forces — sometimes in alliance with ­Jabhat al-Nusra — battling the army of Assad.

The Islamic State has been steadily advancing westward along the Syria-Turkey border.

The U.S. training initiative, begun several months ago in Turkey, has had difficulty recruiting Syrian opposition members for a program that is funded to train 5,000 of them.

The first contingent of about 60 was sent into Syria on July 14, with new equipment and vehicles. U.S. officials said the fighters had pledged allegiance to a goal of fighting against the Islamic State, rather than becoming involved in the many other battles underway in the region.

DeYoung reported from Washington.

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