French air strikes in Syria may have killed French jihadists, a source close to Prime Minister Manuel Valls said Monday, although the defence ministry said the information could not yet be confirmed.

The strikes Thursday night targeted a training camp for Islamic State group militants in the conflict-torn country -- the second time that French jets have targeted IS camps in Syria.

"The strikes have killed jihadists. There might well be French jihadists among them," said the source in Valls's delegation during a visit to Jordan as part of a Middle East tour.

"A figure of six dead was announced, probably by a Syrian non-governmental organisation," he said.

The defence ministry acknowledged Monday that French citizens or French speakers may have been among those killed in the camp, which reportedly lies five kilometres (three miles) southwest of Raqa, the Syrian town widely considered to be IS's centre of power.

But the ministry added it could not "confirm with accuracy anything related to this bombing."

"We know that this camp aimed to train fighters to come and attack Europe and France," the ministry told AFP.

- 'Terrorists don't have passports' -

Valls himself did not confirm any French deaths.

"Our responsibility is to strike Daesh and we will continue to do so regardless of their nationalities," he said in the Jordanian capital Amman, using an Arabic acronym for IS.

"The terrorists, from that point of view, don't have passports."

Valls denied however that there had been any "bungling" in the apparently different accounts.

The Syrian Observatory for Human Rights, a monitoring group with a vast network of contacts in Syria, said it had no information that French citizens had been killed in the attack on the camp.

On Friday, the Observatory said 16 people had been killed in the strikes, including one leading jihadist named Abu Abdullah al-Belgiki, who is nicknamed "The Belgian" and is presumed to have Belgian nationality.

Later in the visit to Jordan, Valls went to an airbase used by French fighter jets that are targeting IS in Iraq.

French authorities have stepped up efforts to fight radicalisation since a series of jihadist attacks by gunmen left 17 dead in January in Paris.

France has been named as a priority target by IS militants and several other attacks have been foiled. Fears are high that battle-hardened jihadists could return from Iraq or Syria to launch attacks on French soil.

French intelligence believes that of the approximately 1,000 French jihadists who have travelled to Syria at some time, 139 have been killed there.

France, like the United States and Britain, says it is acting in self-defence by striking jihadists in Syria.

Although Britain is not carrying out air strikes in Syria, it used a drone to kill 21-year-old Reyaad Khan, a suspected jihadist believed to have been planning attacks on British soil, in Raqa in August.

The British government faces a potential legal challenge over the strike from opposition politicians and legal rights charity Reprieve and France fears it too could face legal action if French citizens are killed in air strikes.