US air raids in Syria overnight targeted and likely killed a French bombmaker who was a leader of an Al-Qaeda offshoot accused of plotting attacks on the West, a defense official said Thursday.

David Drugeon was a Muslim convert suspected of working with Al-Qaeda veterans in what Washington calls the Khorasan group, which American officials say is a dangerous militant outfit planning to attack the United States and other Western countries.

US officials have portrayed Drugeon, 24, as a dangerous figure and his death will be touted as a serious setback for the Khorasan group.

"He was among the targets," a senior US defense official who spoke on condition of anonymity told AFP. "We think we got him."

But it will take time to confirm his death with absolute certainty, particularly as there are no US troops deployed on Syrian soil, the official added.

It was only the second strike against the Khorasan group since September 23, when US-led air raids began in Syria. In that operation, Tomahawk cruise missiles targeted the Khorasan militants but US officials said it was unclear if key leaders had been taken out then.

The US military's Central Command, which oversees the air war against the Islamic State group in Iraq and Syria, said warplanes carried out air strikes against Khorasan militants, vehicles and locations used to produce homemade bombs, but did not say if Drugeon was hit.

US aircraft struck Khorasan again because they have "a history of training operatives, facilitating fighters and money, and planning attacks against US and Western targets," Central Command said in a statement.

"We're still assessing the results of those strikes," General Lloyd Austin, head of Central Command, said in Washington.

Asked if Drugeon was targeted, Austin said: "He is clearly one of the leadership elements, one of those dangerous elements in that organization. And so any time we can take their leadership out, it's a good thing."

-- Football to jihad --

While touted as a small victory, the air raids illustrated the difficulties Washington faces as it tries to carry out its fight against the IS -- and other extremists -- while avoiding being drawn into Syria's complex civil war and a direct confrontation with the Damascus regime.

The US military made a point of saying the air strikes against Khorasan "were not in response" to recent fighting in which Al-Nusra militants have pushed back moderate rebel forces.

The strikes "did not target the Nusra Front as a whole," Central Command said.

They "were directed at the Khorasan group, whose focus is not on overthrowing the Assad regime or helping the Syrian people," it said. "These Al-Qaeda operatives are taking advantage of the Syrian conflict to advance attacks against Western interests."

Born into a middle-class French family, Drugeon was an ardent football fan as a boy before becoming steadily radicalized along with his brother following his parents' divorce in 2002, adopting the name "Daoud" instead of David.

Former classmates and local families have express shock at his transformation into a hardline Islamist.

After saving his money, Drugeon traveled to Egypt to attend a religious school and to learn Arabic. After returning to France, he told his family in 2010 that he was returning to Egypt, but in fact headed to the tribal areas of Pakistan to join extremists in international jihad.

He soon learnt how to handle explosives and make bombs.

A security source who asked not to be identified said he became a "relatively good" explosives expert.

French officials have insisted Drugeon has no links to the country's military or intelligence services, as some US media alleged.

France is not taking part in the US-led air campaign in Syria but has conducted strikes in Iraq as part of the international coalition against the IS group.