A convoy of pro-government fighters entered Syria's Afrin region on Tuesday to support Kurdish fighters battling Turkey's military but immediately came under artillery fire.

Syrian state television showed video of the convoy of pro-government forces deployed to help fend off Turkey's assault against Kurdish YPG fighters.

"The Syrian government has responded to the call of duty and sent military units on this day ... to deploy along the border and take part in defending the unity of Syria's territory and borders," YPG spokesman Nouri Mahmoud said in a statement.

State TV showed about 20 vehicles with heavy weapons mounted entering Afrin from the nearby village of Nubul. Dozens of armed men were on the vehicles waving Syrian flags and chanting pro-government slogans.

Rami Abdel Rahman, head of the UK-based war monitor Syrian Observatory for Human Rights, confirmed "hundreds of fighters entered the Afrin region on Tuesday afternoon".

Hasty retreat?

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Turkish forces fired "warning shots" at the Damascus-backed fighters as they entered the region towards Afrin city on Tuesday.

"Pro-regime terrorist groups that are trying to advance towards Afrin retreated to about 10 kilometres [six miles] from the town because of the warning shots," Turkish state news agency Anadolu said.

Syrian news agency SANA confirmed Turkish artillery fire but made no mention of any retreat.

"Turkish regime forces targeted the locations of popular forces with artillery fire as they arrived to the Afrin region," SANA reported.

The shelling marks a serious escalation in the month-old assault Turkey and allied rebels are waging on Afrin.

Mevlut Cavusoglu, Turkey's foreign minister, said on Monday his military would hit back if pro-Syrian forces intervened in Afrin to help the YPG.

Ankara sent its military into northern Syria last month saying it needed to defeat the YPG to protect its border.

YPG, part of the US-backed Syrian Democratic Forces umbrella organisation, gained control of large swaths of territory in northern Syria during an offensive against Islamic State of Iraq and the Levant (ISIL).

US support for the YPG has infuriated Turkey as it considers the group a "terrorist" organisation.

Ankara sees YPG as part of the banned Kurdistan Worker's Party (PKK), which has waged a decades-long bloody rebellion against the Turkish state in southeastern parts of the country.