US-led air strikes in Syria hit Al-Qaeda-linked militants and an Islamist rebel brigade in a rare expansion of weeks-long raids targeting the Islamic State group, a monitor said Thursday.

American media reported that 24-year-old French bomb-maker David Drugeon, a Muslim convert who joined an Al-Qaeda offshoot, the Khorasan group, was killed in the overnight strikes.

"We think we got him," a senior US defence official told AFP, speaking on condition of anonymity, adding that it would take time to confirm his death with absolute certainty.

The raids hit Al-Qaeda affiliate Al-Nusra Front for only the second time since the US-led coalition began bombings in the war-torn country on September 23, the Syrian Observatory for Human Rights said.

Strikes near the Turkish border also targeted the Ahrar al-Sham rebel group for the first time, the British-based monitoring group said.

The Ahrar al-Sham rebel brigade is believed to have between 10,000 and 20,000 fighters and espouses a conservative ideology, though it has not expressed the same transnational jihadist aspirations as Al-Nusra or IS.

But many of its top leaders have ties to Al-Qaeda and the group has fought alongside Al-Nusra against other moderate rebel groupings in parts of northern Syria.

US media reported that an air strike hit a vehicle in Syria's Idlib province, believed to be carrying Drugeon.

Fox News, citing "well-placed military sources", said the car's driver lost a leg and was not expected to live, while a passenger believed to be Drugeon was killed.

- 'Assessing outcome' -

"We are still assessing the outcome of the attack," Pentagon spokesman Colonel Steven Warren told AFP.

The Syria conflict began as an uprising against President Bashar al-Assad's regime in 2011 and has since escalated into a multi-sided civil war that has drawn thousands of jihadists from overseas.

The fighting has continued despite the US-led raids, with at least 12 people killed in two regime barrel bomb raids in the northern city of Aleppo on Thursday, the Observatory said.

Also Thursday, Syrian troops backed by pro-regime militiamen recaptured the Shaer gas field in central Homs, state television reported, a week after IS jihadists overran parts of it.

The air strikes against Al-Nusra also killed several jihadists and two children in Idlib province while six jihadists were killed in raids in Aleppo, according to the Observatory, which relies on a network of sources on the ground.

The Al-Qaeda affiliate confirmed the strikes on Twitter, saying they were carried out by "the alliance of Crusaders and Arabs on Al-Nusra positions, causing deaths, mostly of civilians".

On the first day of the US-led campaign in Syria Washington said it had struck a group of Al-Qaeda veterans that it called "Khorasan", although analysts said the militants belonged to Al-Nusra.

Until now there had been no other reported strikes on groups outside IS.

The latest raids came after Al-Nusra made gains against Western-backed rebel fighters in the Idlib region.

In a statement posted on Twitter, Ahrar al-Sham also confirmed its base near the Bab al-Hawa border crossing had been hit, saying the raids "benefit only the criminal (Syrian) regime".

- Seeking 'advanced weapons' -

Speaking to Lebanon's Al-Akhbar newspaper, meanwhile, Syria's Foreign Minister Walid Muallem said Washington had pledged that the coalition strikes would not target the Syrian military.

"Do we trust this commitment? For now. We realise that President Barack Obama, for domestic reasons, wants to avoid war with Syria," Muallem said.

"But we do not know how Obama will act under mounting pressure, and the pressure will only increase if the Republicans achieve a majority in the US mid-term elections, so we have to prepare ourselves."

The interview was conducted before Tuesday's US elections, which saw the Republicans regain control of Congress.

Muallem said Damascus had "bluntly" explained its concerns to Moscow, asking them to "provide us with advanced weapons".

He said delivery of S-300 anti-aircraft missiles was now "just around the corner".

S-300 batteries are advanced surface-to-air weapons that can take out aircraft or guided missiles.

Russia has remained a staunch ally of Assad throughout the conflict that began in March 2011, and the country's TASS news agency reported Thursday that Muallem would visit Moscow later this month.