Turkey considers the YPG to be an extension of the Kurdistan Workers' Party (PKK), which has waged a deadly insurgency against the Turkish state since 1984 and is listed as a terror group by Turkey, the USA and the EU.
Turkish military advances into northern Syria over the past two years have put USA forces directly in the path of advancing troops from Turkey, Washington's main Muslim NATO ally.
"We have been in communication (with) both Turkey and the SDF to de-escalate the situation", spokesman Colonel Sean Ryan said on Twitter, referring to the Syrian Democratic Forces.
Syrian authorities have, for their part, repeatedly opposed the presence of U.S. and Turkish forces in Manbij, with the Syrian Foreign Ministry describing the situation in Manbij as "a violation of United Nations resolutions". The campaign against ISIS "is not over and that fight remains hard", he said.
The escalation comes a day after Turkish President Recep Tayyip Erdogan told his party lawmakers that the military was preparing to launch an operation against Kurdish forces east of the Euphrates River. The Syrian Observatory for Human Rights said on Thursday that at least 100 more Kurdish fighters had arrived in the area less than 24 hours after Turkey shelled their positions in the north.
The SDF said Wednesday that it fired back in self-defense, and "as a result, a Turkish military vehicle was destroyed".
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The Britain-based watchdog group said the SDF also sent reinforcement to the frontline as part of the preparations for another round of battles to dislodge IS from its last pocket in the eastern countryside of Deir al-Zour. This assault has already forced Kurdish forces to retreat across the Euphrates river.
Turkey's president said Tuesday his country has finalized plans for a "comprehensive and effective" operation that would target USA -backed Kurdish militia in Syria east of the Euphrates River, a move that could further increase tension in the area where US -led coalition forces are based.
Turkey's state-run news agency says the Turkish military has shelled positions held by US -backed Kurdish fighters across the border east of the Euphrates River in Syria, killing four Kurdish fighters and wounding six others.
To ease ties, the countries agreed to a road map in June that included the YPG withdrawing from Manbij and establishing the joint patrols in the city. Kurdish units liberated Manbij from Daesh* in 2016.
"We are aware of Turkish statements about a planned offensive into northeast Syria and have been in touch with Turkey and the SDF to de-escalate the situation", he added, while saying that the USA military was "working closely with the SDF who are in the midst of offensive operations against ISIS in the Middle Euphrates River Valley", calling the United States allies "a committed partner against ISIS".
Turkey, which sees the Syrian Kurdish groups as a threat to national security, earlier this year threatened military action against Manuja.