Ankara - Turkey's foreign minister on Monday ordered predominantly Kurdish Syrian forces to withdraw east of the Euphrates River "immediately" or face more strikes by Turkish forces that crossed the border last week.Foreign Minister Mevlut Cavusoglu's comments came as Syrian opposition groups reported that Turkish-backed Syrian rebels have captured more towns and villages in northern Syria as part of the operation named "Euphrates Shield," now in its sixth day.Turkish tanks rolled across the border last week to help Syrian rebels seize the town of Jarablus from the Islamic State group, a move that was also aimed at deterring further advances by Kurdish-led forces.The fighting pits Turkey, a Nato ally, against a US-backed proxy that is the most effective ground force battling ISIS militants in Syria's five-year-old civil war. It leaves Washington in the tough spot of having to choose between two allies, and is likely to divert resources from the fight against ISIS.Both Turkey and the United States have ordered the main Kurdish fighting force, the Kurdish People's Protection Units, or YPG, to withdraw to the east bank of the Euphrates."The YPG has to immediately cross east of the Euphrates River as they promised the United States and as they announced they would," Cavusoglu said. "If they don't they will be a target."Earlier this month, the Kurdish-led force known as the Syria Democratic Forces crossed the Euphrates and drove ISIS militants out of Manbij, a key supply hub south of Jarablus, after a 10-week campaign.Both Turkey and the United States have ordered the YPG militia to withdraw to the east bank of the river. YPG leaders say they have, but their units advise the Syrian Democratic Forces, and it is not clear if any remain west of the Euphrates.Syrian opposition activists have said that at least 35 civilians were killed in northern Syria in the Turkish-led operation so far. Turkey denied any civilians had been hit.On Saturday, a Turkish soldier was killed by a Kurdish rocket attack, becoming the first such fatality in Turkey's ground offensive.The Britain-based Syrian Observatory for Human Rights said the Turkey-backed rebels have captured 21 towns and villages near Jarablus from the Syria Democratic Forces.The Observatory also reported clashes on Monday between the rebels and ISIS fighters on the western edge of Jarablus.The Local Coordination Committees, an activist collective, said the rebels captured seven more villages since Sunday.