President Vladimir Putin yesterday ordered his forces in Syria to take tough action against any threats, speaking two weeks after Turkey shot down a Russian warplane in the war-torn country.

"I order you to act as tough as possible," he told a defence meeting.

"Any targets threatening the Russian grouping or our land infrastructure should be immediately destroyed."

"I would like to warn those who would once again try to organise some sort of provocations against our servicemen," he said in a thinly veiled threat to Ankara.

Putin also claimed Russia was backing the Free Syrian Army (FSA) with arms in joint operations with regime forces.

His statement appeared to be the first time Moscow said it was actually supporting Syrian President Bashar al-Assad's opponents in the fight against Islamic State forces. Putin said last month the Russian air force had hit several "terrorist" targets provided by the Free Syrian Army.

"We are supporting them (the FSA) from the air as well as the Syrian army, assisting them with weapons, munition and materiel."

It was not immediately clear what rebel groups Putin was referring to. Syria's rebel forces have regularly rejected the possibility of cooperating with the regime or Russia and there has been no evidence of such cooperation.

Last month, Turkey shot down a Russian warplane on the Syrian border, claiming it violated Turkish airspace.

After the downing of the jet, Russia introduced economic sanctions against Turkey and beefed up its firepower at its airbase in Syria.

In Riyadh, Syria's main opposition groups agreed at unprecedented talks to negotiate with President Bashar al-Assad's regime but insisted he step down at the start of any political transition. But yesterday Secretary of State John Kerry said that some problems remain to be resolved in the pact without specifying any points.

On the ground, a triple truck bomb attack carried out by Islamic State in northeastern Syria killed at least 50 people and wounded 80 others, a spokesman for the Kurdish militia that controls the area said yesterday. The three blasts, carried out by at least two suicide bombers, hit outside a hospital, at a market and in a residential area in the town of Tel Tamer late on Thursday, the YPG's Redur Xelil said via internet messaging service.