Over the weekend, the US moved a small number of troops and several Stryker combat vehicles into Manbij to support the Manbij Military Council, the mostly Syrian Arab group that has controlled the town since the ouster of the Islamic State of Iraq and Syria past year. "So, what I have pledged to our commanders and what I expect from them is for them to ask for the capabilities that they need and then for us to ensure we have the right command and control, the right force protection, the right resources in place to ensure that we can function properly together".
Turkish officials also fear that Assad's forces near Manbij could serve as a blocking force on the Turkish military to prevent its movement south to join an eventual assault on the ISIS stronghold of Raqqa. A US deployment and a Russian-brokered deal with Syrian forces created buffer zones that headed off any Turkish drive against the Kurdish forces - seen by Washington as key allies against Islamic State, though Turkey views them as terrorists - who now hold the town.
"It makes no sense to launch an operation in Manbij without the cooperation of Russian Federation and the United States", Yildirim said Monday.
Before the conflict, Manbij and its surrounding areas had a population of around 600,000.
The last meeting between the two leaders was held in August after the neighbouring countries patched up the fallout from the November 2015 downing of a Russian jet over the Turkey-Syria border.
Ankara views the YPG as a terrorist organization because of its alleged links to the outlawed Kurdistan Workers' Party (PKK), but Washington regards the YPG as an ally in Syria.
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The YPG and local militia forces in Manbij say the YPG subsequently left the city, though Turkey says that is not true.
The group said it delivered food and other supplies including clothes and blankets to between 350,000 and 500,000 Syrians across the border in Syria each month.
Elsewhere in northern Syria, government forces and their allies marched closer to a main water pumping station controlled by IS in Aleppo province, Syria's military and an activist group said.
The so-called Syria Democratic Forces (SDF), which is led by the YPG, captured Manbij in August 2016 with the help of the USA military.
Turkey is cooperating with Russian Federation in Syria, despite them being at odds politically over the future of Syrian President Bashar al-Assad, whom Moscow backs militarily.
The country's president, Recep Tayyip Erdogan, said last week the next target for the operation in northern Syria was Manbij. The U.S. -led coalition "has taken this deliberate action to reassure coalition members and partner forces, deter aggression and keep the focus on defeating ISIS", spokesman Col. John Dorrian said on Twitter.