A convoy of pro-Syrian government fighters entered the Kurdish-held enclave of Afrin in Syria Tuesday before they were targeted by Turkish artillery fire in a dramatic escalation of the conflict in northern Syria.

The convoy of pro-Syrian government fighters in pickup trucks mounted with heavy weapons entered Afrin, where Turkey has been conducting a month-long military operation.

Video clips posted on Twitter showed fighters in camouflage fatigues and waving Syrian flags crossing a checkpoint that bore the insignia of a Kurdish security force.

"One Syria, one Syria!" some of them chanted.

But shortly after they entered the Kurdish-held enclave, the pro-Damascus forces came under Turkish artillery fire and turned back, said Turkish President Recep Tayyip Erdogan.

At a news conference in Ankara, Erdogan said the convoy was made up of “terrorists” acting independently.

"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," said Erdogan.

The Syrian Kurdish YPG (Kurdish Peoples’ Protection Unit) however denied Erdogan’s assertion that the convoy had turned back under Turkish artillery fire.

YPG officials hailed the arrival of the pro-government forces -- which included militias allied to Assad but not the Syrian army itself -- and said Damascus had heeded its call for help against Turkey.

The British-based monitoring group, the Syrian Observatory for Human Rights, said one convoy had entered Afrin while another turned back.

On Monday, Syria’s official news agency SANA said “popular forces” backing Syrian President Bashar al-Assad would enter the Afrin enclave “within hours” to “bolster” local forces confronting Turkish “aggression”.

The claim was denied Monday afternoon by the YPG, underscoring the difficult negotiations between Kurdish forces and the Assad regime.

The latest confrontation pits the Turkish army and their allied rebels directly against the military alliance backing Assad, further scrambling the already messy battlefield in northern Syria.

Erdogan says Turkey will lay siege to Afrin city

Earlier Tuesday, Turkish President Recep Tayyip Erdogan said Turkish forces would "swiftly" lay siege to the centre of the town of Afrin "in the coming days".

His remarks to parliament came as Turkey's operation "Olive Branch", a ground and air offensive against the YPG militia, which Ankara brands "terrorists", entered its second month.

While some analysts say Turkey and pro-Ankara Syrian rebels have made slow advances, Erdogan defended the operation's progress, saying it was to avoid putting the lives of both its troops and civilians needlessly "at risk".

"We did not go there to burn it down," he said, insisting the operation's aim was to "create a safe and livable area" for the Syrian refugees inside Turkey, who fled across the border since the conflict began in 2011 and who now number more than three million.

The UK-based Syrian Observatory for Human Rights monitor said at least 94 civilians have been killed during Turkey's offensive.

But Ankara has repeatedly insisted there have been no civilian casualties, saying its armed forces are showing utmost care not to harm civilians.

Turkey says the YPG is a "terrorist" offshoot of the Kurdistan Workers' Party (PKK), which has waged an insurgency against the Turkish state since 1984.

The PKK is blacklisted as a terror group by Ankara and its Western allies.

But the YPG has been working closely with the United States against the Islamic State group in Syria.

Both Washington and Brussels have urged Turkey to show restraint in its operation, with the US warning the offensive could detract from the fight against jihadists in Syria.