Turkey warned on Wednesday that pro-Damascus forces would face "serious consequences" for entering Syria's Afrin region to help Kurdish fighters repel a Turkish offensive.
Their arrival raises the spectre of wider escalation on Syria's northern battlefront, which includes the Syrian army, allied Iran-linked militias, Kurdish forces, rebels, Turkish troops, and Russian and American forces.
The Syrian Kurdish YPG militia said Turkish planes bombed a town in Afrin and fighting raged on the ground on Wednesday. Turkey launched its assault last month to drive out the YPG, which it deems a menace along its border.
More paramilitary forces aligned with Syrian President Bashar al-Assad went to Afrin on Wednesday, state media said.
Turkish President Tayyip Erdogan's spokesman said shellfire had forced an earlier convoy to retreat. "Any step by the (Syrian) regime or other elements in this direction will surely have serious consequences," Ibrahim Kalin told a news conference.
A commander in the alliance fighting alongside Damascus in Syria's seven-year war told Reuters that pro-government forces in Afrin had returned fire after rebels backed by Turkey attacked them on Tuesday night.
A new confrontation, pitting the Turkish army directly against pro-Assad forces, would further scramble the web of alliances and rivalries already at play in northern Syria.
Erdogan has described the pro-government fighters as Shi'ite Muslim militias acting independently and warned they would pay a heavy price.
Kalin said Turkey was not in direct talks with the Syrian government, but its messages to Damascus were being indirectly conveyed.
"The Syrian forces that entered and are still entering will be in the suitable locations to repel the Turkish occupation army," said Rezan Hedo, a YPG media adviser.
They would deploy near the Turkish border, the YPG has said.
The pro-Assad commander said Russia had intervened to "delay the entry" of Syrian army troops, and so allied "popular forces" with heavy weaponry went instead.
Some Syrian Kurdish officials have said they believed Moscow wanted to keep leverage with Ankara to advance its wider ambitions of brokering a settlement of the conflict.
Turkey and Russia have fought on opposing sides during the seven-year war, with Moscow the key ally of Assad and Ankara one of the main backers of rebels fighting to overthrow him. But Ankara shifted its Syria policy, seeking to mend ties with Russia and turning its firepower against Kurdish forces.
Turkey has in recent months lent support to diplomacy by Russia, whose jets helped Assad's government seize back most major cities since 2015. Ankara said last month it had sought Moscow's agreement before the Afrin assault.
"The Russians are the ones who decided this game," said Kurdish politician Fawza Youssef.
"The Russians have been playing it like this for a while... They pressure the Turks with the Kurdish card (and vice versa)," said Youssef, a senior member of the Kurdish-led autonomous authority in north Syria.
There was no comment from Moscow on Syrian deployments.
Ahead of the Turkish offensive last month, Russia pulled out the military police it had deployed in Afrin last year.
The Turkish offensive was slow to achieve gains along the frontier but pushed several km (miles) into Syria. Still, the YPG holds most of the Afrin region including its central town.
Turkey regards the YPG as an extension of the outlawed Kurdish PKK movement which has waged a three-decade insurgency on its soil, though the groups say they are independent.
In response to questions on Afrin on Wednesday, Russian Foreign Minister Sergei Lavrov urged all Syrian combatants to talk to Damascus within Syria's "territorial intergrity".
Kurdish autonomy declarations in north Syria do "not help to resolve the problem" nor does U.S. support in territory outside government control, the TASS news agency cited Lavrov as saying.
Since the onset of Syria's conflict in 2011, the YPG and its allies have carved out three autonomous cantons in the north, including Afrin. Their power expanded as they fought Islamic State (IS) militants with the help of U.S. jets - though Washington opposes their political ambitions as does the Syrian government.
The U.S. arming of the YPG has infuriated NATO ally Turkey. But while Washington's troops stand with the Kurdish forces in the much larger swathes of east Syria they control, it has not given them support in Afrin.
Syrian Kurdish officials have said they sought assistance from Assad's military after foreign powers did not help them against Turkey's army.
The move of pro-Assad forces into Afrin has drawn more attention to the relationship between Damascus and the YPG, which will prove pivotal in how the conflict unfolds.
The Kurdish fighters and the Syrian government, which each hold more ground than any other side in the war, have mostly avoided direct conflict despite occasional clashes.
But Kurdish leaders seek autonomy as part of Syria, and Assad repeatedly pledges to take back the whole country.
While both have at times hinted a long-term agreement might be possible, their dealings are vulnerable to wider factors as powerful foreign allies also wrangle over Syria.
In Afrin, they have a common enemy in the Turkish forces. Kurdish officials said this week that talks with Damascus over Afrin had remained purely military.
"Until now, there is no talk at all over returning state institutions or administrative matters," said Youssef, the senior Kurdish politician. "The priority now is that there's an assault... After, the political can be discussed."