Al Qaeda’s Syria wing said on Friday it had detained members of a Syrian rebel group who had just returned from US training, in a direct challenge to Washington’s plan to train and equip insurgents to combat the hard-line Islamic State group.

The rest, he said, had deployed back to Syria, but had not been told whether American warplanes would defend them if Syrian forces attacked. “A few days ago, one of these groups called Division 30 entered Syria…so Al-Nusra arrested several soldiers of this division”, the group said in an online statement.

On Friday, Division 30 said Nusra Front fighters attacked its headquarters at 4.30 a.m. (0130 GMT) in the area near Azaz.

The Pentagon as of late Friday was still gathering information about whether and how many fighters were killed or injured.

Abu Thabet said Nusra Front has been referring to Division 30 as “agents of the Americans”.

The Pentagon has denied that any of the initial group of around 60 U.S.-trained rebels known as the “New Syrian Force” had been abducted.

The Nusra Front, which has in the past targeted rebel groups backed by the United States, said coalition warplanes struck their positions in the area of fighting with more than 10 missiles that killed and wounded several fighters.

Division 30’s leaders expected to play a role in an ambitious new joint push by the United States and Turkey to help less radical Syrian insurgent groups seize territory from the fundamentalist militant fighters of the Islamic State.

Eight of them, including a commander, were kidnapped on Thursday by Al-Nusra in a village in Aleppo province, the Observatory said.

Syrian activists posted images of what they said were coalition strikes on an al-Nusra command center in Azaz.

A spokesman for the American military, Col. Patrick S. Ryder, wrote in an email statement that “we are confident that this attack will not deter Syrians from joining the program to fight for Syria”, and added that the program “is making progress.”

“Coalition forces struck multiple Daesh (IS) targets in the vicinity of the eastern Syria border to reduce the freedom of movement of Daesh”, said Brigadier General Kevin Killea, chief of staff of the US-led coalition.

The Nusra Front said in a statement on Friday that its aim was to eliminate Division 30 before it could gain a deeper foothold in Syria. The Observatory said 18 Nusra Front fighters were killed in the fighting and the airstrikes.

A Pentagon program to train moderate Syrian insurgents to fight the Islamic State has been vexed by problems of recruitment, screening, dismissals and desertions that have left only a tiny band of fighters ready to do battle. By staying out of the fight, they may have signaled that they have not accepted a central feature of the Pentagon’s program: that it be directed only at the Islamic State and not at the Syrian government forces of President Bashar Assad, against whom the rebels originally took up arms.

The Obama administration has long struggled to find partners on the ground in Syria to work with in its war against the IS group. The Pentagon has said however that no members of the “New Syrian Force” had been captured or detained. And it makes a mockery of recent administration claims that the U.S. and Turkey have agreed to set up an ISIS-free zone in northern Syria.

More than 230,000 people have been killed in Syria since the conflict started in March 2011 with anti-government protests.