Kurdish fighters battled to defend a key Syrian border town from Turkish-backed forces on Tuesday as Russia seized on a US withdrawal to move its troops into new areas.
The Damascus regime dispatched more forces to contain the Turkish advance, but its key ally Moscow vowed to prevent clashes between the two sides.
Washington slapped sanctions on its NATO ally in a bid to stop an assault triggered by its own abrupt pullout, but Turkish President Recep Tayyip Erdogan said the operation would continue until "our objectives have been achieved".
The Syrian regime's move was its most significant deployment in the Kurdish-controlled region since it started withdrawing troops from the area in 2012.
It followed a deal announced Sunday between Damascus and the Kurds that saw regime troops raise the Syrian flag in the flashpoint northern city of Manbij on Tuesday after US forces withdrew.
Regime troops had entered the city near the Turkish border late Monday, as Turkey-backed fighters massed to the west ahead of a planned assault.
Kurdish fighters from the Syrian Democratic Forces (SDF) are battling Turkish forces and their Syrian proxies -- mostly former opposition factions paid and equipped by Ankara -- in various parts of the northeast.
East of Manbij, the Kurds are mounting a desperate defense of the border town of Ras al-Ain, using tunnels, berms and trenches.
An AFP correspondent in the area said clashes around the town continued Tuesday, despite Ankara's repeated claims it had captured the area.
The Britain-based Syrian Observatory for Human Rights said Kurdish fighters had launched "a large counterattack against Turkish forces and their Syrian proxies near Ras al-Ain".
A Turkish-backed Syrian fighter said heavy artillery attacks by pro-Turkish forces were facilitating the advance.
Erdogan vows to continue
Since launching their assault on October 9, Turkish-backed forces have secured more than 100 kilometers (60 miles) of border, but Ras al-Ain -- Siri Kani in Kurdish -- has held out.
President Donald Trump announced a pullback of US forces from the border last week, a move widely interpreted as green-lighting a long-planned Turkish invasion.
Erdogan, who like Trump faces political difficulties at home, wants to create a buffer zone stretching 30 kilometers (20 miles) from the border into Syrian territory.
He wants to keep at bay the SDF -- seen in Ankara as "terrorists" linked with Kurdish insurgents in Turkey -- as well as resettling some of the 3.6 million Syrian refugees currently in Turkey.
"God willing, we will quickly secure the region stretching from Manbij to our border with Iraq," Erdogan said Tuesday.
The offensive has killed dozens of civilians, mostly on the Kurdish side, and displaced at least 160,000 people.
The UN Security Council will meet Wednesday to discuss Turkey's incursion.
Aid groups, which have warned of a new humanitarian disaster in Syria's eight-year war, have pulled out international workers and halted work, the Kurds said, but UN agencies continue to operate.
Trump, who is campaigning for re-election but faces impeachment, is keen to deliver on a promise to pull US troops out of Syria.
The US will withdraw more than 1,000 troops from northern Syria, keeping only a residual contingent of around 150 at the al-Tanf base near the southeastern borders with Jordan and Iraq.
Russia's defense ministry said US troops had left bases near Manbij and the Syrian regime "has full control" over the former ISIS group stronghold.
The US-led coalition confirmed the withdrawal, saying: "We are out of Manbij."
US troops had been based in the city since helping Kurdish fighters seize it from ISIS in 2016.
Russian military police continue to patrol a zone separating regime troops and Turkish forces, in cooperation with Ankara, Moscow said.
Russia's special envoy on Syria, Alexander Lavrentiev, said Turkish and Syrian officials were in contact to avoid any clashes between Turkish and Syrian forces.
"This would simply be unacceptable," Russian news agencies quoted him as saying.
Turkey said two of its soldiers were killed on Tuesday in "terrorist" artillery fire in the Manbij region, bringing its death toll to six since the start of the offensive.
Transfers to Iraq?
Abandoned by the US -- their chief ally in years of battles against ISIS that cost the lives of 11,000 of their fighters -- the Kurds turned to Damascus.
European governments are worried the chaos could trigger mass breakouts by thousands of ISIS fighters detained by Kurdish forces.
They have warned this could lead to an ISIS resurgence and increase the risk of some extremists returning to Europe and conducting terror attacks.
The SDF have warned their fighters were mobilized to counter the Turkish advance and not available to fully guard ISIS prisoners.
The Kurds have said hundreds of ISIS relatives escaped, although Trump suggested the SDF may have deliberately released them to gain leverage.
At least three French women escapees were "retrieved" by ISIS, according to messages they sent to their lawyer seen by AFP.
Human Rights Watch warned European countries against attempts to have their detained nationals transferred en masse to Iraq for prosecution.
The watchdog warned it would be illegal to send them to a country where due process is consistently violated and they risk execution.
A European diplomatic source said French and Iraqi officials were discussing a transfer on Tuesday.