France rejects 'unfair' Moscow criticism of UN Syria envoy

France has 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.

The comments from Paris come after Russian Foreign Minister Sergei Lavrov lambasted UN envoy Staffan de Mistura on Monday for failing to restart peace talks aimed at ending the brutal five-year conflict.

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.

"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 on Tuesday.

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

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

But the ceasefire has produced little respite, with ongoing rebel rocket attacks on government-held Aleppo killing dozens of civilians and air strikes by the govenrment and its Russian ally reported around Aleppo, the Syrian Observatory for Human Rights monitor said.

Russian strikes on Palmyra

On Tuesday the Russian defence ministry also announced that six of its bombers had struck targets in and around Palmyra, killing "a great number" of IS militants.

Six Tupolev bombers flew out from a base in Russia on Tuesday morning and "made a concentrated strike with fragmentation bombs on (IS) facilities" east of Palmyra and the towns of Sukhna and Arak, the defence ministry said on Facebook.

Russia gave advance warning to the US-led coalition, which is conducting a separate bombing campaign in the war-torn country, the ministry said.

IS was forced out of Palmyra by Syrian government forces in March, but Arak and Sukhna, both seized in the spring of 2015, remain in the rebels' hands.

Arak, located 35 kilometres (21 miles) east of Palmyra, is a small town that has strategic importance because of a nearby oil field, while Sukhna, 70 km northeast of Palmyra, is an IS bastion.

Russia said its strikes targeted sites that were "recently detected and confirmed through several intelligence channels."

The raids killed "a great number of personnel" and destroyed a field camp as well as three ammunition depots, three tanks and a dozen other vehicles, Moscow said.

IS’s propaganda arm, the Amaq Agency, had earlier claimed that the group’s militants were active in the south of the city, destroying a Syrian army missile launcher.

Reports of a renewed IS advance towards Palmyra come after IS shot down a Russian helicopter in the area on Saturday, killing two pilots.

The defence ministry said a Syrian helicopter flown by Riafagat Khabibulin and Yevgeni Dolgin "came under fire" from IS on Saturday and crashed.

Speaking on Tuesday Ayrault also repeated an aid pledge made in April by French President Francois Hollande, with Paris due to provide Lebanon with $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.