ANKARA/BEIRUT — Pro-Syrian government forces entered Syria’s northwestern Afrin region on Tuesday to help a Kurdish militia there fend off a Turkish assault, raising the prospect of a wider escalation of the conflict.

Soon after the convoy of militia fighters – waving Syrian flags and brandishing weapons – entered Afrin, Syrian state media 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 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 this.

A commander in the pro-Assad military alliance told Reuters the forces had turned back after coming under fire, but then resumed their progress and were now in Afrin.

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.

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 Iranian President Hassan Rouhani, Assad’s main international backers, to block Syrian government support for the YPG fighters.

The commander in the pro-Assad alliance said Russia had intervened to “delay the entry of a large (number of) Syrian army forces” into Afrin.

Erdogan described the pro-government fighters coming to the YPG’s aid as Shia militias, and said they would pay a heavy price.