The United States said it conducted air strikes on Wednesday night against the so-called Khorasan group, an al Qaeda-linked militant faction based in Syria, and said the group was plotting to attack Europe or the United States.
US officials, speaking on condition of anonymity, said a target of the strike was David Drugeon, a French-born militant and convert to Islam who some US officials say is a bomb maker for the group.
General Lloyd Austin, the head of the US military's Central Command, said Drugeon was one of the group's "leadership elements and one of the most dangerous elements in that organisation."
He declined to say whether Drugeon was killed, telling a forum in Washington the military was assessing the results of the strikes. Asked whether Drugeon was a target, he said, "Any time we can take their leadership out is a good thing."
The officials said they believed a leader of the Khorasan group, Muhsin al-Fadhli, who had been targeted in US strikes in Syria in September, was still alive. It was unclear whether al-Fadhli was a target of the latest US raid.
In a statement on Thursday, US Central Command said the latest strikes were carried out by the US military against five Khorasan targets near Sarmada in Idlib province, close to the Turkish border and west of the Syrian city of Aleppo.
"We took decisive action to protect our interests and remove their capability to act," it said, adding that al Qaeda militants "are taking advantage of the Syrian conflict to advance attacks against Western interests."
"SKILLED AL QAEDA VETERANS"
US officials have described Khorasan as a grouping of skilled al Qaeda veterans who moved to Syria from the Afghanistan-Pakistan border and set up operations under the protection of Nusra Front, the main Syrian al Qaeda affiliate.
From strongholds in northwestern Syria, Nusra Front has fought militants in the Islamic State, another spin-off of al Qaeda which holds territory in Syria and Iraq and is considered a major threat in the area by Washington.
The Syrian Observatory for Human Rights, a monitoring group, said a series of US air strikes targeted Nusra Front in Idlib province, where last week the group pushed back Western-backed Syrian rebels. The Observatory said at least six Nusra militants had been killed.
There was no independent confirmation that this was an account of the same attack described by CENTCOM.
The US military made clear the attacks were specifically aimed at Khorasan and not more broadly at Nusra Front. "There were no strikes conducted against al Nusra," Austin said.
US officials have described Khorasan as a particularly menacing faction of militants who have been using their sanctuary in Syria to try to organise plots to attack US and other Western targets, possibly including airliners.
(Reporting by Mark Hosenball, Phil Stewart and Susan Heavey; Additional reporting by David Alexander in Washington and Suleiman Al-Khalidi in Amman; Editing by David Storey, G Crosse and Lisa Shumaker)