Turkey plans to launch a new military offensive against Kurdish militants east of the Euphrates river in northern Syria "in a few days," a move that raises the risk of confrontation with the United States, Turkish President Recep Tayyip Erdogan said Wednesday.

Erdogan made the announcement hours after the US confirmed setting up "observation posts" in northern Syria along the border with Turkey. Ankara has long objected to the posts.

"Our target is never the American soldiers, but the members of the terrorist organization active in the region," Erdogan said at a defence industry summit in Ankara.

But the Pentagon on Wednesday issued a stern warning against any unilateral military moves in north-east Syria.

"Unilateral military action into north-east Syria by any party, particularly as US personnel may be present or in the vicinity, is of grave concern," Pentagon spokesman Sean Robertson said in a statement.

"We would find any such actions unacceptable," Robertson added.

Erdogan said the Turkish operation against the People's Protection Units (YPG), a Syrian Kurdish militia backed by the US, would start in a few days east of the Euphrates river.

Ankara believes the YPG and the Kurdish Democratic Union Party (PYD), its political arm in Syria, are tied to the outlawed Kurdistan Workers' Party (PKK), which has waged a decades-long insurgency within Turkey.

There are US troops stationed in northern Syria. While Turkey controls areas west of the Euphrates, an offensive in the east would be a first.

Erdogan said the operation - Turkey's third in Syria - will help "maintain the territorial integrity of Syria and pave the way for a political solution."

Erdogan accused the US for using Islamic State as an "excuse," insisting that its backing of the YPG, which controls large areas of northern Syria on the border with Turkey, was no longer justified.

"There is no longer any such threat as Daesh (Islamic State) in Syria. For us this is a fairy tale," Erdogan said, adding that if Washington wanted help to eliminate Islamic State, it could ask Ankara.

The Turkish president expressed the intention of cooperating with the US in the east of the Euphrates and in Manbij, which the YPG seized from the Islamic State group in 2016. The US and Turkish forces began joint patrols in those two areas in November.

YPG spokesman Nouri Mahmoud told dpa by phone that the group's forces were ready to confront any possible attack on the eastern Euphrates.

"Erdogan is trying to show the world that he is willing to change the map in the area," Mahmoud said.

Turkey shelled YPG positions in northern Syria at the end of October. Erdogan at that time threatened to "destroy" Kurdish armed groups east of the Euphrates despite a US military presence in the region.

The Turkish army and allied Syrian rebels captured the YPG-controlled Kurdish enclave of Afrin in north-western Syria earlier this year.

Late Wednesday, the PYD denounced Erdogan's announcement of the forthcoming offensive.

"It is a new incident to create obstacles when there is a small hope of solving the Syrian crisis," a PYD statement said, adding that Erdogan's move was aimed at preventing the expulsion of Islamic State militants from their remaining strongholds.

"Ankara is trying to end the stability and co-existence in northern and eastern Syria," the statement said.

The PYD called on the United Nations, the UN Security Council, the Syrian regime, Arab League, the world community and all Kurdish parties "to stand united to foil the goals of Erdogan."