Turkey shells Assad loyalists trying to cross into Syria's Afrin

Turkish forces shelled Syrian pro-government militia fighters on Tuesday as they entered the northern canton of Afrin to aid the Kurdish YPG force in its battle against a Turkish incursion.

Turkish media reported its forces struck areas around the pro-Damascus volunteers as they moved from the towns of Nubl and Zahraa to the YPG-controlled enclave.

Soon after the convoy of militia fighters - waving Syrian flags and brandishing weapons - entered Afrin, Syrian state media also reported that Turkey had targeted them with shellfire.

The confrontation pits the Turkish army and allied Syrian rebel groups directly against the military alliance backing the government of President Bashar al-Assad, further scrambling northwest Syria's already messy battlefield.

Turkish President Recep Tayyip Erdogan described the convoy as being made up of "terrorists" acting independently. He said Turkish artillery fire had forced it to turn back, although the Kurdish militia denied his claim.

Syrian television had earlier shown the group of fighters passing through a checkpoint that bore the insignia of the Kurdish security force, some chanting "one Syria, one Syria", and driving further into Afrin.

Erdogan said the pro-government fighters would "pay a heavy price".

"This evening some pickups were proved to be moving to Afrin but then artillery made then retreat and that file is closed now," he said.

The YPG hailed the arrival of the pro-Assad forces, which did not included the Syrian army itself, and said they were deploying along the front line facing the Turkish border.

The Baqir Brigade said on Facebook it would be leading pro-government forces "against FSA terrorists and Turks" in Afrin, in reference to rebels fighting on Turkey's behalf.

The move comes in spite of an apparent promise extracted from the Russian president, Vladimir Putin, by his Turkish counterpart that there would not be any Syrian army presence in Afrin.

Speaking after a speech in parliament on Tuesday, Erdogan said: "The [Syrian deployment] was seriously stopped yesterday... It was stopped."

Erdogan on Monday had spoken to his Russian and Iranian counterparts, Putin and Hassan Rouhani, to discuss developments in Syria and Afrin.

"Yes, it was stopped after those talks,” Erdogan said.

Although the NDF is not a part of the main Syrian army, it does receive government weaponry and fights alongside regular forces.

Video released following the strike on the troops appeared to indicate that some were retreating in the face of the attacks.

Anadolu Agency, without giving a source, said the forces had turned back some 10km.

Erdogan announced on Tuesday that a Turkish military siege on the northern Syrian town of Afrin was imminent, leading to increased fears that civilians in the densely populated urban centre could be at extreme risk.

"In the coming days quicker actions will be taken to lay siege to Afrin's city centre,” Erdogan told his party's MPs in parliament.