Turkey will launch a new military operation against U.S.-backed Kurdish fighters in Syria "within a few days," President Recep Tayyip Erdogan said Wednesday, a move likely to further strain ties between NATO allies Turkey and the United States.

In recent months, Turkey has shelled Kurdish positions across the border in Syria, east of the Euphrates River, and has threatened to drive out the Syrian Kurdish militia known as the People's Protection Units, or YPG.

The YPG is the main component of a Kurdish-led militia in Syria that rolled back the Islamic State group with the help of the U.S.-led coalition. Ankara views YPG fighters as terrorists because of their links to the Kurdish insurgency inside Turkey. U.S. troops are deployed with the Kurdish fighters in northeastern Syria, in part to prevent clashes with Turkey.

"We will begin our operation to rescue the east of the Euphrates from the separatist organization within a few days," Erdogan said. "Our target is not the American soldiers, it is the terror organizations that are active in the region."

He called on the U.S. not to allow "deep" disagreements over Syria policy to impede future cooperation between the two countries.

Turkey has been incensed by American support for the Kurdish-led forces. More recently, it has been frustrated by delays in the implementation of an agreement that was reached between Washington and Ankara that would effectively push the Kurdish militia out of the key northern Syrian town of Manbij, west of the Euphrates.

"They are not being honest, they are still not removing terrorists (from Manbij)," Erdogan said. "Therefore, we will do it."

Erdogan's statement, given at a defense industry meeting, follows U.S. moves to set up observation posts in northern Syria, despite Turkey's objections. Turkey says the observation posts are aimed at protecting the YPG, while the U.S. says the posts were established to address Turkey's security concerns.

"We know that the aim of the radar and observation posts set up by the United States (east of the Euphrates) is not to protect our country from terrorists, but to protect the terrorists from Turkey," Erdogan said.

Commenting on the Erdogan remarks, a Pentagon spokesman said U.S.-Turkish coordination and consultation is the "only approach" to security issues in this area.

"Unilateral military action into northeast Syria by any party, particularly as U.S. personnel may be present or in the vicinity, is of grave concern," Navy Cmdr. Sean Robertson said. "We would find any such actions unacceptable."

Robertson said uncoordinated military operations will undermine the shared U.S.-Turkish interests in security in Syria.

"The campaign against ISIS is not over," he said. "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. The SDF remains a committed partner against ISIS and we remain committed to working with them to ensure ISIS's enduring defeat. 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."

On Tuesday and Wednesday, Turkey sent convoy of troops and military equipment to the border with Syria as reinforcements, according to the state-run Anadolu Agency.

Turkish troops have already driven back Syrian Kurdish forces in two cross-border operations west of the Euphrates, in 2016 and earlier this year.

Meanwhile, the Britain-based Syrian Observatory for Human Rights, a war-monitoring group, said at least four people were killed in two explosions in Syrian towns held by Turkey-backed local administrations shortly after Erdogan's announcement.

A motorcycle exploded near a mosque in the town of al-Bab, killing three people. The second explosion occurred in the town Azaz, where one child was killed, the Observatory said.

There were other, smaller explosions in al-Rai and Dabiq, it said. No one was reported killed in those blasts.

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Associated Press writer Robert Burns in Washington contributed to this report.