Turkey will launch a new operation against US-backed Kurdish militias in northern Syria "in the next few days", President Recep Tayyip Erdogan says.
The move, which he said would focus on territory to the east of the Euphrates river, risks confrontation with the US.
America's support for the Kurdish YPG forces has strained relations with Turkey, which considers the YPG to be part of a terrorist group.
Turkey has launched two offensives against the Kurds in Syria since 2016.
The Kurdish People's Protection Units (YPG) currently controls large swathes of the north-east of Syria on Turkey's southern border.
What did President Erdogan say?
"We will start the operation to clear the east of the Euphrates from separatist terrorists in a few days," Mr Erdogan said on a televised speech on Wednesday, referring to territory held by the YPG.
He did not specify which areas would be targeted, but it would be the first time Turkish troops have moved east of the river.
"Our target is never US soldiers," he added. There are some 2,000 US troops currently in Syria, many of them stationed in the north.
Turkey considers the YPG an extension of the banned Kurdistan Workers' Party (PKK), which has fought for Kurdish autonomy in south-eastern Turkey for three decades.
Mr Erdogan wants to prevent the Kurds from consolidating their hold on Syrian territory and forming an autonomous region on the border.
What's the context?
US soldiers have been working closely with Kurdish forces who form part of an alliance - the Syrian Democratic Forces (SDF) - in the battle against the Islamic State (IS) group. Turkey has long criticised this US policy.
Tensions between the two sides have risen in recent weeks. Turkey says it is frustrated over what it sees as delays to a deal agreed with the US to clear a flashpoint city in the north of Kurdish fighters.
The deal over Manbij, which lies to the west of the Euphrates river, was agreed in February in a bid stabilise the region.
And on Tuesday, the Pentagon announced it had erected observation posts in the northern border region aimed at preventing clashes between the Turkish army and Kurdish fighters.
Turkey's Defence Minister Hulusi Akar has called on the US to scrap the move and end its co-operation with the YPG.
In late October, Turkey shelled Kurdish militia posts in northern Syria, forcing the SDF to briefly suspend its operations against IS.
What has Turkey already done in Syria?
Turkey has launched two major offensives in recent years in northern Syria. Both took place west of the Euphrates river.
The first - dubbed Euphrates Shield - began in the summer of 2016 and was an eight-month operation targeting IS and Kurdish forces that ended in March 2017.
Earlier this year they launched a second military operation - Olive Branch - against Kurdish militia in Afrin province. It lasted two months and the city was eventually cleared of the Kurds.
However Turkish forces have until now avoided direct confrontation with Kurdish fighters and their US backers located on the east of the Euphrates river.