ANKARA, Turkey — Turkey sent more tanks into northern Syria on Thursday and gave Syrian Kurdish forces a week to scale back their presence near the Turkish border, a day after it launched a US-backed cross-border incursion to establish a frontier zone free of the Islamic State group and Kurdish rebels.
Skirmishes broke out between Turkish-backed Syrian rebels and the US-backed Kurdish fighters south of the contested town, raising the potential for of all-out confrontation between the two American allies that would also jeopardize the fight against the Islamic State in the volatile area.
Turkey’s incursion Wednesday to capture the town of Jarablus was a dramatic escalation of the NATO nation’s role in Syria’s war and adds yet another powerhouse force on the ground in an already complicated conflict.
But Ankara’s objective went beyond fighting extremists. Turkey is also aiming to contain the expansion by Syria’s Kurds, who have used the fight against Islamic State and the chaos of Syria’s civil war to seize nearly the entire stretch of territory along Syria’s northern border with Turkey.
Above all, Ankara seeks to avoid Kurdish forces linking up their strongholds along the border. The United States has backed its NATO ally, sending a stern warning to the Syrian Kurds with whom it has partnered in the fight against the Islamic State to stay east of the Euphrates River. The river crosses from Turkey into Syria at Jarablus.
‘‘The US is interested in stopping this from becoming a confrontation between the YPG and Turkey. That would be a huge detriment to the anti-IS campaign,’’ said Chris Kozak, a Syria researcher at the Washington-based Institute of the Study of War, referring to the main US-backed Kurdish faction fighting the Islamic State. Turkey accuses the group of links to Kurdish groups waging an insurgency in southeastern Turkey.
Kozak said an open confrontation between Turkey and the Kurds in Syria would undo much of the progress made working with the Kurdish forces against the Islamic State in northern Syria. If there are direct clashes, the US would be forced to take sides, he said, and Washington would likely side with its NATO ally, whose air base is used to launch coalition airstrikes against the extremists in Syria and Iraq.
Also, if the Syrian Kurdish forces are distracted in clashes with the Turks and have to shift resources toward front lines with Turkey or with Turkish-backed opposition groups, that ‘‘buys [the Islamic State] some breathing space,’’ Kozak said.
On Thursday, Turkish officials said Syrian Kurdish forces had started withdrawing east of the Euphrates. The news was relayed by Secretary of State John Kerry in a telephone conversation with his Turkish counterpart, Mevlut Cavusoglu, said the officials, who spoke on condition of anonymity in line with government regulations.
Syrian Kurdish officials contacted by The Associated Press would not confirm or deny that their forces were withdrawing east. Instead, the main Syrian Kurdish faction, the YPG, said its troops had ‘‘returned to their bases’’ after helping liberate the northern Syrian city of Manbij from the Islamic State earlier this month. Manbij lies west of the Eurphrates, and Ankara has demanded the Kurds hand it over to Syrian rebels and withdraw.
The Kurdish forces’ statement said they handed control of the city to a newly-established Manbij Military Council, made up mostly of rebel fighters from the town.
By daybreak, at least 10 more Turkish tanks crossed into Syria, Turkey’s private Dogan news agency reported. An Associated Press journalist saw three armored vehicles cross the border, followed by a heavy construction vehicle. Explosions reverberated across the border, followed by billowing gray smoke.
It remained unclear whether Turkey-backed Syrian rebels would move against Islamic State-held towns or nearby Kurdish-controlled areas, including the town of Manbij.
Turkey’s state-run Anadolu agency, reporting from Jarablus, said the Turkish-backed Syrian opposition forces were working to secure the town to allow its resident’s to return, including defusing explosives inside the town or on roads leading to it. Estimates put the town’s population at 25,000.
Turkey’s defense minister, Fikri Isik, said Thursday that Turkish forces were securing the area around Jarablus. He said the Turkish-backed operation had two main goals — to secure the Turkish border area and to make sure the Syrian Kurdish forces ‘‘are not there.’’