"The political track is the only way forward," Ayrault said on the second day of a visit to Syria's neighbour.

"If things are blocked today, this is not the fault of Staffan de Mistura. They are blocked because the ceasefire has broken down, and today Aleppo is under siege and a hostage," French Foreign Minister Jean-Marc Ayrault told reporters in Lebanon.

The UN-backed talks, aimed at ending a war that has left more than 280,000 people dead and driven millions from their homes, are in theory set to resume this month.

On Monday, Russian Foreign Minister Sergei Lavrov lambasted UN envoy Staffan de Mistura for failing to restart peace talks aimed at ending the brutal five-year conflict.

France on Tuesday rejected as "unfair" Moscow's criticism of the UN envoy on Syria, calling for a ceasefire and lifting the Aleppo siege to allow the peace process to resume.

"If we want Staffan de Mistura to fully carry out his mission, the siege of Aleppo must be lifted, there must be a ceasefire and access for humanitarian aid must be allowed."

Lavrov on Monday said De Mistura "is shunning his duties and not convening the next round of inter-Syrian talks".

But the UN envoy has said he wants guarantees of progress before restarting the process.

"The Russians are part of the solution," Ayrault said on Tuesday in the Lebanese capital.

"If the Damascus regime did not have the backing of Russian air power it would not have had the means to do

what it has done.

"If they sincerely want the peace process to resume, there must be a ceasefire as soon as possible, as well as the siege of Aleppo being lifted," he added.

"I think it is very unfair to Staffan de Mistura, who is carrying out his mission to do everything to create the conditions for a peace process through negotiations," Ayrault said, adding that "going down the path of controversy and caricature is wrong".

On Tuesday, Syrian regime forces extended a nationwide truce for another three days, but at the same time pressed their campaign against rebels in the battered northern city.

Smoke billows following a reported strike by pro-Syrian government forces in a rebel-held neighbourhood in the northern city of Aleppo on July 12, 2016 (photo by: Karam al-Masri/AFP)

But the ceasefire has produced little respite, with ongoing strikes by the regime and its Russian ally reported around Aleppo, the Syrian Observatory for Human Rights monitor said.

Ayrault repeated an aid pledge made in April by French President Francois Hollande, with Paris due to provide Lebanon with 50 million euros ($55 million) by the end of 2016.

Earlier this year, Saudi Arabia suspended a grant to finance $3 billion worth of French weaponry for Lebanon's security forces.

The tiny Mediterranean country is struggling to cope with 1.1 million registered Syrian refugees.

It has been without a president since May 2014 when Michel Sleiman's mandate expired, and parliament has extended its own mandate twice since 2009.

"We aim to create the conditions to help you exit this crisis," Ayrault said after discussions with several Lebanese officials.