BAGHDAD — A Syrian insurgent group at the heart of the Pentagon’s effort to fight the Islamic State came under intense attack on Friday from a different hard-line Islamist faction, a serious blow to the Obama administration’s plans to create a reliable military force inside Syria.

The American-led coalition responded with airstrikes to help the American-aligned unit, known as Division 30, in fighting off the assault, according to an American military spokesman and combatants on both sides. The strikes were the first known use of coalition air power in direct battlefield support of fighters in Syria who were trained by the Pentagon.

The attack on Friday was mounted by the Nusra Front, which is affiliated with Al Qaeda. It came a day after the Nusra Front captured two leaders and at least six fighters of Division 30, which supplied the first trainees to graduate from the Pentagon’s anti-Islamic State training program.

In Washington, several current and former senior administration officials acknowledged that the attack and the abductions by the Nusra Front took American officials by surprise and amounted to a significant intelligence failure.