US air strikes killed 50 Al-Qaeda militants and eight civilians, including children, in northern Syria on Tuesday, the Syrian Observatory for Human Rights said.
Most of the 50 fighters killed in the attacks west of the second city Aleppo were foreigners, and the civilians included three children and one woman, the Britain-based monitoring group said.
The strikes came in addition to separate raids carried out by a US-led coalition including Arab nations that hit the Islamic State jihadist group in north and east Syria.
In Washington, the Pentagon confirmed eight US strikes against "seasoned Al-Qaeda veterans" in Aleppo province.
"The United States has also taken action to disrupt the imminent attack plotting against the United States and Western interests conducted by a network of seasoned Al-Qaeda veterans -- sometimes referred to as the Khorasan Group," it said in a statement.
The group, it added, has "established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations".
"These strikes were undertaken only by US assets," the statement added, unlike the strikes against IS.
The Khorasan Group is believed to refer to an initiative by Al-Qaeda's central command in the Afghanistan-Pakistan region to establish a group in Syria of some of its veteran militants to focus on attacks against the West.
Its members cooperate with Al-Nusra Front, Al-Qaeda's Syria affiliate, using the group's resources and bases, according to experts.
Al-Nusra's focus, however, has so far been on the fight against President Bashar al-Assad's regime, and its members are largely Syrian citizens.