The US says some of its air strikes in Syria aimed to disrupt an imminent attack on the West by al-Qaeda veterans from the so-called Khorasan group.
The raids in Aleppo province came hours after the US, backed by Arab allies, launched the first air strikes against Islamic State (IS) militants in Syria.
The Aleppo raids were carried out by the US alone, the US military said.
The Khorasan group was in "the final stages of plans to execute major attacks", a US military spokesman said.
Lt Gen William Mayville, the Pentagon's operations chief, said the group's targets were "either in Europe or the homeland".
Other officials have said the group was planning to attack aircraft.
Very little is known about Khorasan outside of recent statements by US officials.
After the latest air strikes in Syria, the US military said the group had "established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations".
On Tuesday, US President Barack Obama confirmed that the group had been targeted in the overnight strikes on Syria.
"We will not tolerate safe havens for terrorists who threaten our people," he said.
Analysis: Peter King, BBC Monitoring
The term Khorasan Group, apparently coined by the US, is used to describe a group of al-Qaeda veterans who have travelled to Syria to take advantage of the situation there.
Embedded within al-Qaeda's regional branch there, al-Nusra Front, and possibly other local groups, they have come largely from the Afghanistan and Pakistan region, which jihadists refer to as Khorasan.
The US says they have been plotting an imminent attack against US and Western interests, but the cell itself has not said anything about its aims or existence. Jihad supporters online believe the US is using the phrase to refer to operatives within al-Nusra Front or the group itself.
Several al-Qaeda veterans have turned up in Syria this year, including al-Nusra Front's spokesman, Abu Firas al-Suri, and its top military commander, Abu Humam al-Suri. Both have appeared in al-Nusra Front propaganda.
But the Khorasan cell's alleged leader, Muhsin al-Fadli, has had no public profile.
The US Central Command said earlier on Tuesday that fighter aircraft had carried out eight air strikes targeting Khorasan to the west of Aleppo.
At a Pentagon briefing later in the day, a spokesman said that more than 40 Tomahawk missiles had been launched, the majority them targeting Khorasan. The missiles are fired from ships or submarines.
There has been no confirmation of the impact of the air strikes on the ground.
Some 50 al-Qaeda-linked militants and eight civilians are reported to have been killed in overnight US raids on Syria's Aleppo and Idlib provinces.
It is not clear if the casualties - reported by the UK-based Syrian Observatory for Human Rights, an activist group that monitors the Syrian conflict - are linked to the US strikes targeting Khorasan.
Most of the 50 dead militants are said to have been foreign citizens. The civilian deaths reportedly included one woman and three children.