Turkish-backed Syrian forces have reinforced the area around the town of Manbij, a rebel spokesman said on Monday, as they prepare for a withdrawal of US troops following Washington's surprise decision to pull out of Syria.
The heightened military activity comes as Ankara and Washington have agreed to coordinate on the US withdrawal. Outgoing US Defence Secretary James Mattis signed the order for the withdrawal of the US forces from Syria, the defence department said on Monday.
US President Donald Trump's abrupt decision has upended American policy in the region and set Turkey up for a broad push against Syrian Kurdish YPG forces across its border.
Differences over Syria have been a longtime source of tension between the two NATO allies. The United States has backed the YPG in the fight against Islamic State, but Turkey sees the militia as a terrorist organisation tied to the outlawed Kurdistan Workers Party (PKK).
"Yesterday units from the Syrian National Army headed towards the Manbij front and took preliminary positions (in preparation) for the battle," said Major Youssef Hamoud, the spokesman for the National Army, the main Turkey-backed rebel force in the area. The group is aimed at unifying disparate factions in northwest Syria.
US forces still remain in Manbij - a flashpoint for tension just west of the Euphrates in northern Syria - and the Turkey-backed fighters will not advance until they withdraw, Hamoud told Reuters.
In June Turkey and Washington reached an agreement that would see the YPG ousted from the town, but Turkey has repeatedly criticised the United States for what it said was a delay in that deal.
President Tayyip Erdogan said earlier this month that Turkish forces would enter the town if the United States does not remove the Kurdish fighters.
While officials have cautioned against a timeline, the withdrawal is seen as possibly beginning within weeks. Erdogan has said Ankara will postpone a planned military operation against the YPG east of the Euphrates river in northern Syria following the US decision to pull out.
Erdogan and Trump agreed in a phone call on Sunday to coordinate to prevent an authority vacuum from developing as the United States withdraws from Syria, the Turkish presidency said.
On Sunday, a Reuters witness saw hundreds of vehicles head to the southern border province of Kilis after leaving Hatay, another border province. The convoy included tanks, howitzers, machine guns and buses carrying commandos, the witness said.