"Unilateral military action into northeast 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," Cmdr. Sean Robertson, a Defense Department spokesman, told CNN.
Earlier on Wednesday, Anadolu Agency reported military vehicles loaded with equipment traveling to the Turkey-Syria border in the Elbeyli region of Kilis province, on the west side of the Euphrates River.
"We believe this dialogue is the only way to secure the border area in a sustainable manner, and believe that uncoordinated military operations will undermine that shared interest," added Robertson.
Turkey previously has threatened to conduct cross-border operations against the predominately Kurdish Peoples' Protection Units (YPG). The force forms the backbone of the US-led international anti-ISIS coalition and has been described by the US military as the most effective force against the extremists.
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.
The SDF reported notable gains against the last ISIS stronghold east of the Euphrates in the Hajin pocket over the weekend for the first time in about six weeks.
Turkey claims the YPG are the Syrian offshoot of the Kurdistan Workers' Party (PKK), a political party banned by Turkey and designated as a terrorist organization by Ankara, Washington, and Brussels. YPG denies the claims.
To try to assuage Turkey, the coalition conducts joint patrols with Turkish forces west of the Euphrates around Manbij. The US military also reportedly began regular patrols along the border with Turkey to ease Ankara's concerns over fighters and arms flowing across the border in early November.
In his speech on Wednesday, Erdogan said their target "definitely" is not US troops.
"It is the members of terror organization operating in the region. I want to emphasize this," Erdogan added.
Coalition forces from the United Kingdom, France, and the United States have been seen in northern Syria. They work extremely closely with the SDF the facilitate air strikes against ISIS, establish local military councils, and help to clear mines, IEDs, and unexploded ordinance from liberated areas to allow displaced Syrians to return.
"The campaign against ISIS is not over. Coalition forces are working closely with the Syrian Democratic Forces who are in the midst of offensive operations against ISIS in the Middle Euphrates River Valley," said Robertson from the Pentagon.
Erdogan had claimed that ISIS is no longer a threat in northern Syria during his speech on Wednesday.
"We should not and cannot allow ISIS to breathe at this critical point or we will jeopardize the significant gains we have made alongside our Coalition partners and risk allowing ISIS to resurge," he added.
