Turkish President Recep Tayyip Erdogan announced on Wednesday that the armed forces "will begin our operations to save the east of the Euphrates from the separatist terrorist organization within a few days", referring to an upcoming campaign against the People's Protection Units (YPG).
The PKK is blacklisted as a terror group by Ankara and its Western allies.
Washington, keen to protect its interest in the area, has -somewhat hypocritically- warned Ankara against launching a "unilateral" military campaign that might endanger USA soldiers, who have recently established observation posts along the Turkish-Syrian border.
There are about 2,000 US troops on the ground, USA defense officials say.
A USA official, speaking on condition of anonymity because discussions between the countries aren't public, said that unilateral military action in that part of Syria by any party would be unacceptable, and that the only way to address security issues of mutual concern is through collaboration between Turkey and the U.S.
Elizabeth Teoman, analyst at the Institute for the Study of War (ISW), said Erdogan may be threatening the attacks "to compel a change in United States policy regarding the U.S. observation posts along the Syrian-Turkish border". "We decisively want to bring peace and tranquility to civilians in the east of the Euphrates, as we have already done in other regions of Syria", the president said.
American forces have worked closely with the YPG under the Kurdish-led Syrian Democratic Forces (SDF) alliance against the Islamic State (IS) group. Turkey regards the YPG as an extension of the PKK.
"Erdogan has lost hope that the US would stop its support for SDF, particularly since the USA observation points along the Syria-Turkey border have become a reality", said John Saleh, a Syrian affairs analyst in Washington.
In a bid to avoid any clash, the North Atlantic Treaty Organisation allies agreed a "roadmap" for Manbij in June.
Analysts in Turkey say Erdogan's recent comments regarding the YPG shouldn't be taken lightly.
Erdogan, however, said Turkey had not got the result it wanted in Manbij, adding, "There has been a delaying tactic undeniably used in Manbij, and right now it is still being used".
Hammoud said the different rebel groups "were informed a while ago" of a possible operation, adding that training supervised by Turkish officers had been underway. The Turkish currency has been among the world's worst performers this year, partly due to escalating tensions with the USA on a range of issues, from Syria policy and Erdogan's growing ties with Russian Federation to Turkey's detention of an American pastor.
The YPG were pulled away from the battlefield twice before - once when the Turkish army and their allied Syrian militias invaded Afrin and then again last month when Turkish forces fired on Kurdish positions and villages near the border. Earlier this year, it conducted an offensive against the Kurdish militia in Syria's northern border city of Afrin.
Erdogan has repeatedly warned Turkey would not allow "a terror corridor" to be formed on its border by the YPG.
Erdogan claimed that there are American soldiers in the detachments of the terrorists in Syria.