While the world has focused on a US-led air assault on Islamic State strongholds in Syria, American officials said they also struck a blow there against a little-known cadre of hardened al Qaeda militants that posed a more immediate threat to the West.
The strikes early on Tuesday on what Washington called the Khorasan Group, so shadowy that United States officials had rarely uttered its name in public, were staged to disrupt a plot against US or European targets that the Pentagon said was "nearing the execution phase".
The US objective may also have been to take out the leader of the cell, Kuwaiti-born Mohsin al-Fadhli, a reputed former member of Osama bin Laden's inner circle, reports say.
Despite Islamist posts on social media mourning Fadhli's death, there was no confirmation that he was among the dozens reported killed in the bombing raids in north-western Syria.
The strikes followed lengthy surveillance of Khorasan, described by US officials as a "network" of seasoned al Qaeda fighters with battlefield experience mostly in Pakistan and Afghanistan and now working in league with al Qaeda's Syrian affiliate, the Nusra Front.
The al Qaeda veterans have used the chaos of Syria's civil war as cover to try to devise new hard-to-detect bombs and recruit foreign militants holding Western passports to return home and eventually carry out attacks, US officials said.
The Islamic State, which also has al Qaeda roots and flourished during Syria's civil war, has pressed a brutal campaign to establish an Islamic caliphate centred in parts of Syria and Iraq, but is not seen by Washington as an immediate threat outside the region.
US officials said the small Khorasan group, on the other hand, has pursued the single goal of plotting bombings in the United States and Europe.
Concern about such plots prompted US authorities to order tighter airport screening for flights to the United States in July. Airlines were asked to give closer scrutiny to passengers' cellphones and shoes, apparently fearing they could be used to conceal explosives.
After Tuesday's air strikes, the Pentagon said it was still assessing how badly Khorasan had been hit, but a senior US official said the bombing was "quite effective".
Islamist militants on social media said there were unconfirmed reports that 33-year-old Fadhli had been killed and they were mourning him. But the US official said, "We don't have confirmation on that leadership target."
According to a 2012 State Department notice that offered a $7 million reward for information on his whereabouts, Fadhli was an al Qaeda financier close to Osama bin Laden and among the few who knew in advance about the 11 September 2001 attacks in New York.