A convoy of pro-Syrian government fighters arrived in Syria's northern region of Afrin to fight Turkey. - AFP

Turkish forces shelled a column of pro-Assad regime fighters as they tried to join with Kurdish forces resisting a Turkish incursion into northern Syria, Turkey’s resident Recep Tayyip Erdogan said.

The regime has been saying for days that it would dispatch fighters to support the Kurds as they battle to keep Turkey from taking control of the Kurdish-held pocket of Afrin.

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.

Mr 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 this.

"Unfortunately, these kind of terror organisations take wrong steps with the decisions they take. It is not possible for us to allow this. They will pay a heavy price,” he said.

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.

Pro-regime fighters drive past a banner of the Kurdish leader Abdullah Ocalan Credit: GEORGE OURFALIAN/AFP/Getty Images

Ankara's month-old offensive is aimed at driving the Kurdish YPG militia, which it sees as a big security threat on its border, from Afrin.

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

It made no mention of a deal that a Kurdish official said on Sunday had been struck with Assad's government for the Syrian army itself to enter Afrin.

Erdogan said he had previously reached an agreement with Russian President Vladimir Putin and Iran's President Hassan Rouhani, Assad's main international backers, to block Syrian government support for the YPG fighters.

He described the pro-government fighters coming to the YPG's aid as Shi'ite militias, and said they would pay a heavy price.

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YPG media adviser Rezan Hedo denied Erdogan's assertion that the convoy had turned back under Turkish artillery fire, but he gave no details on its size or composition. A Britain-based monitoring group, the Syrian Observatory for Human Rights, said one convoy had entered Afrin while another turned back.

President Recep Tayyip Erdogan said his forces had driven back regime troops Credit: Pool Photo via AP

Earlier on Tuesday, Erdogan said he had received Putin's agreement to block a Syrian government deployment in Afrin.

Turkey and Russia have supported opposite sides throughout the war, with Moscow the closest ally of Assad and Ankara one of the principal supporters of rebels fighting to overthrow him.

However, in recent months Turkey has lent support to a Russian-led effort to end the war with most population centres in the hands of Assad's government. Ankara said last month it sought Moscow's agreement before launching the Afrin assault.

Russian Foreign Minister Sergei Lavrov said on Monday the Afrin crisis could be resolved through direct negotiations between Damascus and Ankara.

Assad's other main ally, Iran, is more closely involved than Russia with the militias that back the Syrian government on the ground, such as those who entered Afrin on Tuesday.

The Turkish offensive has made gains along almost all the border area with Afrin, pushing several km (miles) into Syria and seizing villages. But the YPG still holds most of the region including its main town, also called Afrin.