US-led aircraft bombed Islamic State group (ISIS or IS) fighters battling rival rebels including Al Qaeda loyalists in northern Syria overnight, a monitoring group said on Sunday.
The Syrian Observatory for Human Rights described the raids in Aleppo province as intervention on the side of the rival rebels, even though they include forces that have previously been targeted by US-led strikes.
"The coalition carried out at least four strikes overnight targeting IS positions in the town of Suran," the Britain-based group said.
"It's the first time that the international coalition has supported non-Kurdish opposition forces fighting the Islamic State," Observatory director Rami Abdel Rahman told AFP.
He said at least eight IS fighters were killed in the strikes.
IS seized control of Suran a week ago, and has since been fighting an alliance of Islamist rebels including Al Qaeda affiliate Al-Nusra Front and the Ahrar al-Sham movement in the surrounding area.
Both Al-Nusra and Ahrar al-Sham have been targeted in US-led raids, including as recently as May 20, when a strike in Aleppo province killed 15 Al-Nusra fighters.
Like IS, Al-Nusra is blacklisted as a "terrorist organisation" by Washington.
IS has been seeking to expand its territory by seizing the Aleppo province towns of Marea and Azaz, which lie on key supply routes for the rival rebels.
Online, IS supporters accused Al-Nusra and Ahrar al-Sham of collaborating with the US-led coalition, denouncing them as "America's spies."
Despite sharing a jihadist ideology, Al-Nusra and IS are fierce rivals, with IS seeking to expand its self-declared caliphate in territory it holds in Syria and Iraq.
Al-Nusra pledges allegiance to Al-Qaeda chief Ayman al-Zawahri, but has for now confined its ambitions to Syria and has allied with conservative Islamist groups fighting the regime and IS.
The US-led coalition began its air campaign in Syria last September but the majority of its strikes have been confined to areas where IS has undisputed control or is battling Kurdish forces.
Washington has ruled out any coordination or cooperation with the Damascus government over its air campaign.