French President Francois Hollande called on Saturday for international action against an al-Qaeda affiliate in Syria, warning that the recent losses sustained by the Islamic State (IS) group could embolden other militant groups.

"Daesh [an Arabic acronym for IS] is in retreat, that is beyond dispute," Hollande said after a meeting with the leaders of the US, Germany, Britain, Italy and Ukraine on the sidelines of a NATO summit in Warsaw.

But Hollande added: "We must also avoid a situation whereby as Daesh becomes weaker other groups become stronger."

Hollande singled out al-Qaeda-linked al-Nusra Front as particularly standing to benefit from the US-led military campaign against its arch-rival IS.

Faced with a barrage of air strikes and ground offensives by local forces, IS has lost territory in both Syria and Iraq in recent months.

"We must coordinate among ourselves to continue actions against Daesh but also ... take effective action against al-Nusra," Hollande said, directing his appeal at Russia and the US.

On Wednesday, US President Barack Obama and his Russian counterpart, Vladimir Putin, agreed in a telephone call to "intensify" military coordination between their two countries in Syria.

Russia had in May proposed joint air strikes with the US against militant targets in Syria, a suggestion that was rebuffed by Washington.

The White House reported that the two leaders in their call this week "confirmed their commitment to defeating ISIL [IS] and the al-Nusra Front".

The two groups are excluded from a broader truce brokered by Moscow and Washington in February.