The US called on Turkey to not follow through on threats to attack US-backed forces in Syria Wednesday, an incursion the Pentagon warned could threaten US personnel and derail the fight against ISIS.
"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," Commander Sean Robertson, a spokesman for the Department of Defense, told CNN.
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"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," he added.
Earlier on Wednesday, Turkey's President Recep Tayyip Erdogan said in a televised speech that Turkish troops would launch a military operation east of the Euphrates in Syria, an operation aimed at targeting Kurdish militants.
The US troops in Syria regularly work with Kurdish elements of the Syrian Democratic Forces as part of their campaign against ISIS.
Turkey sees all Kurdish forces in in Northern Syria as an extension of the Kurdistan Workers' Party (PKK), listed as terror group by the US and the EU. The US does not share Turkey's view.
"Our target is definitely not American troops. It is the members of terror organization operating in the region. I want to emphasize this," Erdogan added, saying that he expected the operation to begin "in a couple of days."
Secretary of Defense James Mattis recently directed US troops to establish a series of observation posts in the northeast Syria border region as part of an effort to reduce tensions between Turkey and the US-backed Syrian Democratic Forces.
Two US officials told CNN that while the US has observed additional Turkish forces arriving in the area, at this point in time it is assessed that Turkey does not have enough troops in the area to conduct the type of operation in the timeline outlined by Erdogan.
However the threat of cross-border shelling remains, potentially putting US troops there at risk.
Previous cross-border clashes caused the Syrian Democratic Forces to suspend their hard-fought offensive against the ISIS-held town of Hajin, the terror group's last remaining redoubt east of the Euphrates River.
"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," Robertson, the Pentagon spokesman said.
"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.