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

"Daesh (the Arabic acronym for Islamic State) is in retreat, that is beyond dispute," Hollande said after a meeting with the leaders of the United States, 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 the Islamic State group.

Faced with a barrage of airstrikes 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.

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".

Meanwhile, Syrian regime forces have repelled a fierce assault by opposition fighters seeking to reopen their only supply route into Aleppo city, killing at least 29 rebels, a monitor said yesterday.

The offensive sought to push regime forces back from the Castello Road that leads into the opposition-held eastern half of Aleppo city, which is now effectively besieged by government troops.

The Syrian Observatory for Human Rights said 29 fighters from the Islamist Faylaq al-Sham rebel group and al-Qaeda affiliate Al-Nusra Front were killed in fighting or by mines laid by government troops.