Turkish President Recep Tayyip Erdogan said on Saturday that Ankara will carry out an air and ground military operation east of the Euphrates River in Syria "as soon as today or tomorrow".
Speaking at the opening of his AK Party's annual camp, he said Turkey aimed to "water the east of Euphrates with fountains of peace" and settle refugees there.
His remarks were the most direct indication of an incursion since Ankara and Washington decided to establish a "safe zone" in the area.
"We gave all warnings to our interlocutors regarding the east of Euphrates and we have acted with sufficient patience," Erdogan said.
The operation could start as soon as Saturday or Sunday, he said.
The US-backed force which controls the region, the Syrian Democratic Forces (SDF) led by the Kurdish People’s Protection Units (YPG), said it wanted stability but vowed to respond to any attack.
"We will not hesitate to turn any unprovoked attack by Turkey into an all-out war on the entire border to defend ourselves and our people," SDF spokesman Mustafa Bali said.
NATO allies, Washington and Ankara, have agreed to establish a zone on the Syrian border that Turkey says should stretch 30 km (19 miles) into Syria and be cleared of the YPG, which it considers a terrorist organization.
Turkey has accused the United States, which supports the YPG-led force that defeated ISIS in Syria, of moving too slowly to set up the zone. The two countries are also at odds over how far it should extend into Syria and who should control it
Turkey says it wants to settle up to 2 million Syrian refugees in the zone, and has repeatedly warned of unilateral military action if it is not satisfied with progress.
The SDF, which has said it will pull back up to 14 km (8.7 miles) on some parts of the border, is committed to the agreements under US-Turkish talks and will remain so if "dangerous threats" stop, its spokesman Bali said.
Kurdish commanders have warned that a Turkish border attack would lead to a resurgence of ISIS.
"Simply, there will be a big gap in the towns that our forces will withdraw from to go to defend the border, and this will give ISIS the opportunity to return," Bali said.
Turkey-backed Syrian opposition factions pledged on Friday to back a potential cross-border offensive that Ankara has threatened to mount against Kurdish fighters in northeast Syria.
Turkey has twice launched military offensives with the factions in northern Syria in recent years.
A Turkish military operation in northeastern Syria would carry a larger risk, bringing troops into a zone where at least 1,000 US soldiers are deployed.