Turkish President Recep Tayyip Erdogan on Sunday said Ankara was "seriously saddened" by footage showing US military vehicles operating close to the border with Syrian Kurdish fighters, threatening further military action against a group Turkey sees as "terrorists".
His comments came amid rising tensions over the weekend along the border, with both Ankara and Washington moving armoured vehicles to the area.
Turkish forces last week carried out deadly air strikes on military positions belonging to the Syrian Kurdish People's Protection Units (YPG), angering the US and sparking days of border clashes with the Kurdish fighters.
The US sent military vehicles with American flags to the Syrian side of the frontier accompanied by YPG fighters to carry out patrols, in an apparent bid to prevent further fighting.
Turkey views the YPG as a "terrorist" organisation and an extension of the Kurdish PKK group that has waged a three-decade insurgency against Turkey for greater independence.
But Washington views YPG as its most effective ground partner in the fight against Islamic State of Iraq and the Levant (ISIL) in Syria.
Pictures posted by pro-Kurdish activists on Twitter show military convoys flying US and YPG flags passing through the mostly Kurdish-controlled city of Qamishli.
Turkey's president was unimpressed by the images.
"Unfortunately ... the presence of an American flag along with the [insignia] of a terror organisation called YPG in a convoy has seriously saddened us," Erdogan told reporters in Istanbul, before embarking on a trip to India.
Al Jazeera's Hashem Ahelbarra, reporting from Gaziantep along the Turkey-Syria border, said US troops have patrolled the area in what appears to be an attempt to de-escalate the situation between the Turkish military and YPG.
"Now the Americans face the delicate task of trying to answer some of the concerns of the Turkish government, while at the same time maintaining the YPG as a key component in the fight against ISIL," Ahelbarra said.
The difference of opinion over YPG has cast a shadow over US-Turkish relations for some time, and Erdogan is hoping for a drastic change in American policy when he meets President Donald Trump next month.
"We will bring this up when we meet Mr President on May 16," said Erdogan.
He expressed regret that the US-YPG alliance - which began under President Barack Obama - was being continued under the new US administration.
"This needs to be stopped right now," said 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.
Erdogan reaffirmed that Turkey could again bomb YPG positions any time it wanted.
"I said yesterday: 'We can come unexpectedly in the night'. I really meant that. We are not going to tip off the terror groups and the Turkish Armed Forces could come at any moment.
"Better they live in fear than we have worries," he said.
The private Ihlas news agency reported on Saturday that Turkish military convoys were heading to southeastern Sanliurfa province from Kilis in the west. The base is 50km from Syria's Tal Abyad, a town controlled by YPG.
The agency said the relocation comes after Turkish officials announced the completion of a phase of Turkey's cross-border operation Euphrates Shield in Syria, adding the force may be used against Syrian Kurdish fighters "if needed".
Ankara sent its troops into Syria last August in a military operation triggered in large part by the Kurdish group's expansion along its borders.
Redur Khalil, YPG's spokesman in Syria, said Turkey is reinforcing its border posts opposite Tal Abyad as well as other areas.
"We hope that this military mobilisation is not meant to provoke our forces or for another purpose linked to entering Syrian territories. We don't want any military confrontation between us, since our priority is to fight Daesh [ISIL] in Raqqa and Tabqa," Khalil told The Associated Press in text messages.
Source: Al Jazeera and news agencies