US troops have begun patrolling along the Turkey-Syria border for the first time, after Turkish airstrikes in the area killed allied Kurdish fighters.
Turkey last week bombed targets of the Kurdish People’s Protection Units (YPG) in Syria, a partner force of the US-led coalition but a group considered terrorists by Ankara.
The raid, which left at least 25 fighters dead, earned the wrath of Nato ally Washington as it sees the YPG as the strongest ground force to lead the impending battle for the Islamic State of Iraq and the Levant-held Syrian city of Raqqa.
There are currently more than 500 American special forces and advisers to assist the Syrian Democratic Forces, an umbrella group led by the YPG.
US armoured vehicles were seen along the tense border over the weekend, in an unusual move Washington hopes will discourage Turkey from further strikes.
"The patrols' purpose is to discourage escalation and violence between two of our most trusted (counter-Isil) partners and reinforce the US commitment to both Turkey and the SDF in their fight against ISIS," a statement from the US-led joint command said.
However Turkish President Recep Tayyip Erdogan did little to ease the tension on Sunday night, threatening a repeat.
"We can come unexpectedly in the night," said Mr Erdogan. "We are not going to tip off the terror groups and the Turkish Armed Forces could come at any moment."
US troops on the ground were only a few miles from where the Turkish jets struck, and Washington was reportedly only given a 52-minute warning.
Mr Erdogan said the sight of American flags in the convoy alongside YPG insignia had "seriously saddened" Turkey.
The YPG is the Syrian branch of the Kurdistan Workers' Party (PKK), who have waged an insurgency since 1984 inside Turkey that has left tens of thousands dead.
Mr Erdogan said he would bring up the issue with President Donald Trump during his planned visit to Washington on May 16 -- and called for the coordination between the US and YPG to "come to an end."
The US-YPG cooperation began under the Obama administration, and while Mr Trump said he would rethink the relationship when he took office, little has changed.
"This needs to be stopped right now," said Mr Erdogan. "Otherwise it will continue to be a bother in the region and for us."
"It will also bother us as two Nato countries and strategic partners," he said.
Turkey sent its troops into Syria in August 2016 in support of rebel fighters and as part of an offensive aimed at clearing out Isil and creating a buffer zone between itself and the Kurds.
The International Crisis Group (ICG) said in its latest report on the Syria crisis that the US had "a singular dilemma" on the future of its relationship with the YPG.
It said the YPG "is indispensable" to defeat Isil but there is also "no avoiding the fact" that the US is backing a force "led by PKK-trained cadres in Syria while the PKK itself continues an insurgency against a Nato ally."
It said that Turkey had pressed ahead with the air strikes despite US objections and this "should serve as a warning for what could lie in store."