U.S. airstrikes hit the compound of a powerful hard-line rebel group that is not affiliated to the Islamic State extremist organization early Thursday, activists said, in an apparent widening of targets that could further strain relations between Washington and the Western-backed opposition.

The Syrian rebels have complained that the aerial campaign against the extremist group is indirectly aiding President Bashar Assad's forces, and have been infuriated by the U.S. willingness to attack other Islamic militant groups that the rebels view as allies while refraining from targeting the government.

At least one strike hit a compound belonging to Ahrar al-Sham in the town of Babsalqa in the northwestern province of Idlib, according to local activist Ahmad Kaddour and the Britain-based Syrian Observatory for Human Rights. Ahrar al-Sham is a rebel group that follows an extremely conservative interpretation of Islam.

One of its founders was a senior al-Qaida operative known as Abu Khaled al-Souri. The group is not affiliated with the global terror network, although it has close relations with the Nusra Front, al-Qaida's Syrian branch.

Ahrar al-Sham is part of the Islamic Front, an alliance of seven powerful conservative and ultraconservative rebel groups that merged in November last year. The Islamic Front wants to create an Islamic state in Syria governed by Shariah law and rejects the Western-backed Syrian National Coalition, but cooperates with some of its fighters on the ground.

There was no immediate U.S. comment on the strike.

Activists said the strike was among several that occurred early Thursday in towns along the Syrian border with Turkey.

Other strikes hit compounds of the Nusra Front in the border towns of Sarmada and Harem. The strike in Harem killed at least two children, said Kaddour and the Britain-based Observatory.

The Nusra Front is al-Qaida's affiliate in Syria, but it has fought against the Islamic State.

The U.S. targeted the Nusra Front in the first wave of airstrikes in Syria in late September, accusing it of harboring a militant cell that was plotting attacks against American and Western interests.

Many Syrian rebels view the Nusra Front and Ahrar al-Sham as important allies against both Assad and the Islamic State, and are likely to view U.S. strikes on the two groups as an attack on their nearly four-year-old uprising.

The United States insists it still supports Assad's removal from power but is not targeting government forces.