DAMASCUS, Dec. 19 (Xinhua) -- The U.S. decision to withdraw forces from eastern Euphrates in Syria has shocked the Kurdish-led Syrian Democratic Forces (SDF), a war monitor reported Wednesday.

The U.S.-backed SDF militia was informed that the U.S. forces will withdraw from eastern Euphrates River in northeastern Syria and from the city of Manbij in the northern countryside of Aleppo near Turkey, the Syrian Observatory for Human Rights cited "trustworthy" sources as saying.

The U.S. desicion came as Turkey is planning to renew a military campaign against the Kurdish militia in eastern Euphrates, which has been relying on Washington in its operations against Islamic State (IS) militants in the last IS-held pocket in eastern Euphrates, the Observatory said.

Kurdish leaders said if the United States pulled out from the region, it would be a stab in the back for the Kurdish groups, according to the London-based watchdog.

A day earlier, the Observatory said military movements were seen from both Turkish and Kurdish forces on the border between Turkey and Syria.

In his remarks on Wednesday, Turkish President Recep Tayyip Erdogan reaffirmed Turkey's "decision to wipe out terror groups in the east of Euphrates."

"We will start the operation to clear the east of the Euphrates from separatist terrorists in a few days," Erdogan said, referring to territory held by the Kurdish militias.

The Kurdish self-administration in northern Syria issued a statement last week, calling for mass mobilization in areas under its control and urged the Syrian government to take a stance against the remarks of Erdogan.