The bodies of eight fighters of Harakat Hazm are buried near Aleppo, Syria. They were killed during clashes with Nusra Front fighters on Feb. 28, 2015. (Abdalghne Karoof/Reuters)

The first Syrian rebel group to be given U.S. weapons collapsed Sunday after losing control of its headquarters to Syria’s main al-Qaeda affiliate, further complicating American-led efforts to counter the rise of extremism in Syria.

The rout of Harakat Hazm, whose name means Steadfastness Movement, culminated months of clashes with the al-Qaeda-linked Jabhat al-Nusra in which the moderate group first was pushed from its main headquarters in the northern Syrian province of Idlib and then was ousted Sunday from its new base in the province of Aleppo.

After losing this latest battle, Hazm said in a statement circulated on social media that the movement had been dissolved “in an effort to halt the bloodshed” and that surviving members would be absorbed into a new rebel coalition called the Shamiyah Front.

[Read: Al-Qaeda group’s gains in Syria undermine U.S. strategy]

Nusra fighters boasted on Twitter that they had seized control of U.S.-made TOW anti-tank missiles and other American aid provided to Hazm when they overran the rebels’ headquarters in the town of Atarib in the province of Aleppo. The claims could not be verified, and American supplies of weaponry to moderate rebels in northern Syria had in any case been scaled back in recent months since the battles with Nusra began.

The collapse comes as the Pentagon embarks on a new effort to train moderate rebels to fight the Islamic State, a different extremist group that is at odds with Jabhat al-Nusra and has severed ties with al-Qaeda.

Hazm, which once claimed to have 5,000 fighters, had received U.S. weapons under a separate covert program launched last year by the CIA that was intended to bolster moderate rebels and put pressure on President Bashar al-Assad to compromise with the opposition.

Since Islamic State fighters surged into the Iraqi city of Mosul last summer, the Obama administration has refocused its Syria policy in ways that emphasize defeating the Islamic State rather than pressuring Assad to step down.

[Read: Is it a ‘war’? An ‘armed conflict’? Why words matter]

Hazm’s defeat will muddle the wider effort to combat extremism by leaving large swaths of northern Syria that had once been controlled by moderates in the hands of Nusra, which is not the main focus of the U.S. effort but is formally aligned with al-Qaeda and is also designated as a terrorist group by the United States.

Created a little over a year ago, Hazm earned prominence as the first recipients of U.S.-made TOW anti-tank missiles, the most sophisticated weapons to be supplied directly by the United States and its allies to the Syrian battlefield.

But Hazm and other moderate groups that later received the weapons complained that the quantities were insufficient to make a difference in the fight against the government. And ­after Nusra attacked the U.S.-backed group late last year, the supplies were curtailed, further weakening the moderates.

The new Pentagon program, in contrast, aims to train and equip Syrian rebels to fight only the Islamic State and not the Syrian government. A Pentagon spokesman, Rear Adm. John Kirby, said Friday that the training would begin in the next four to six weeks and that 100 Syrians have been vetted for participation, out of a total intended to exceed more than 5,000 a year.

Syrian rebel commanders say it is unclear whether the program will extend to fighters who have already been vetted and trained by the CIA, such as those in Hazm. It is also unclear whether the fighters who are to be trained under the new program will be charged with fighting Nusra, a fierce rival of the Islamic State that was a target of U.S. airstrikes shortly after they were launched last September but since has been left largely unhindered to expand its control in northwestern Syria.

The Islamic State controls territories in northeastern Syria, where it has suffered a number of battlefield setbacks in recent days inflicted by Kurdish fighters. On Sunday, the Islamic State released 29 of the estimated 90 to 300 Assyrian Christians taken hostage during the battles, Assyrian activists said.