"Unilateral military action into NE Syria by any party, particularly as US personnel may be present or in the vicinity, is of grave concern. We would find any such actions unacceptable," CNN Pentagon correspondent Barbara Starr quoted an unnamed US Department of Defense source as saying.
The US military continues to lead the Global Coalition to Defeat ISIS which is partnered with the ground forces of the Syrian Democratic Forces (SDF), a Kurdish-led force comprised of the Peoples' Protection Units (YPG).
"Coordination and consultation between the US and Turkey is the only approach," the unnamed US source added.
The remarks come amid media close to the SDF announcing that Turkish operations in recent days have distracted from the total defeat of ISIS. The SDF declared ISIS militarily defeated east of the Euphrates on March 23. An undisclosed number of US forces remain embedded and/or working closely with the SDF.
"Following a heavy SDF-led and @Coalition-backed crackdown in the newly-liberated Deir Ezzor region, ISIS are launching more attacks in the heart of the autonomous region. Such attacks are an attempt to sow division among co-existing peoples in North East Syria..." tweeted the Rojava Information Center (RIC).
It later claimed that ISIS attacked in Qamishli, al-Hasaka governorate, for the "eleventh time in eleven days ... which has for some years been secure from ISIS attacks."
SDF fighters repeatedly have said they will defend their northern homelands, when facing attacks there instead of participating in operations further south.
Hawar News, media close to the YPG, announced on Saturday that its Afrin Liberation Forces had clashed with Turkish proxies belonging to Sultan Murad, Ahrar al-Sham, Jabha Shamiya in the al-Bab area.
"Our forces continue to carry out military operations against the soldiers of the Turkish occupation army and its mercenaries in Afrin and its environs, which have killed more and more mercenaries," Hawar News reported.
The US-led international coalition's press office was not immediately available for comment; however, an officer recently categorized operations in Syria.
"Our partners are now progressing steadily… clearing the remnants of #Daesh. The pace is slow and methodical, with the ground and buildings seeded with IEDs and booby-traps, but #progress is being made," the Special Operations Joint Task Force for Operation Inherent Resolve tweeted on July 8, quoting British Maj. Gen. Christopher Ghika.
The account tweeted a day later: "The efforts and sacrifices of partner forces and the #Coalition pay dividends to #safety, #security, and normalcy in #Syria."
Turkey as of Saturday was yet to formally announce any further operations east of the Euphrates, where Manbij has served a de facto demarcation line between it and US forces.
"The latest deployments and tactical mobility on the ground have very strong indications for an offensive east of the Euphrates," Abdullah Agar, a former Turkish military officer and security expert, told Daily Sabah on Thursday.
On Friday, diplomatic tensions between Ankara and Washington again heightened when the Turkish military began receiving its first phase of parts for the Russian-made S-400 air defense system, which is not integrated into NATO.
Both chambers of the US Congress passed versions of the 2020 National Defense Appropriations Act which redirect funding for Turkey's purchase of the fifth-generation F-35 fighter jet.
Turkey began the second phase of Operation Claw against the Kurdistan Workers' Party (PKK) in the Kurdistan Region of Iraq on Friday.
Ankara consider the YPG and SDF to be Syrian offshoots of the PKK, allegations denied by the groups.
