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 state media reported that Turkish artillery had fired warning shots and forced the Syrian fighters back before they reached Afrin.
But Syrian television showed them passing through a checkpoint that bore the insignia of the Kurdish security force, some chanting "one Syria, one Syria", and driving further into the enclave.
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.
But 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 to enter Afrin.
Turkish President Tayyip Erdogan said earlier on Tuesday he had headed off the deployment through a phone call with Russian President Vladimir Putin.
Turkey and Russia have been two of the main outside powers supporting 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 diplomatic effort to end the war with most population centres firmly in the hands of Assad's government. Ankara said last month it had sought Moscow's agreement before launching the assault on Afrin.
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 network of militias - such as those that entered Afrin on Tuesday - that back the Syrian government on the ground.
The Turkish offensive has made gains along almost all the border area with Afrin, pushing several kilometres (miles) into Syria and seizing villages. But the YPG still holds most of the region including its main town, also called Afrin.
"The besieging of the Afrin city centre will start rapidly in the coming days," Erdogan said on Tuesday, adding that this would cut off outside aid.
The entry of pro-government forces into Afrin draws more attention to the uneasy relationship between Assad and the YPG, which each hold more ground than any other forces in the war.
They have mostly avoided direct confrontation during the conflict. But the Kurds seek autonomy in regions they hold, while Assad has pledged to assert control over all of Syria and has called the YPG traitors.
Kurdish political leaders have said they were forced to seek help from Assad's military because no foreign powers would help them against Turkish forces in Afrin.
Russia deployed military police in Afrin last year, but they pulled out last month before the Turkish offensive began.
The United States has armed the YPG as part of the Syrian Democratic Forces (SDF) alliance it backs against Islamic State. US troops are on the ground in other parts of Syria run by the Kurdish-led administration, but not in Afrin.
Washington's support for the YPG has caused deep ructions in its relations with its NATO ally Turkey. In the case of the Afrin offensive, Washington has said it supports Turkey's right to defend itself, while calling for Ankara to show restraint.