"We expect our ground troops to enter the region very soon," Erdogan said of the area east of the Euphrates river, according to state news agency Anadolu.
Turkey and the United States agreed on August 7 to establish a buffer zone near the border with Syria, which has been wracked by civil war since 2011, but few details have been provided about it.
Ankara has been pushing to control an area about 40 kilometres deep in northern Syria and remove US-backed Syrian Kurdish forces there.
Turkey considers these forces - such as the People's Protection Units (YPG), which controls large areas of northern Syria at its border - to be an extension of the Kurdistan Workers' Party (PKK) waging a decades-long insurgency within the country.
"I hope no one will no longer try to test Turkey's determination to clear its border with Syria of terrorists," Erdogan said in the eastern province of Mus.
Washington relied on the Syrian Democratic Forces (SDF), a Kurdish-led militia, as the most effective group in fighting Islamic State in Syria.
The NATO allies' joint operations centre to oversee the so-called "safe zone," is now fully functional and located in Akcakale, in the south-eastern Turkish province of Sanliurfa, which borders Syria.
Turkish Defence Minister Hulusi Akar also said Monday that joint patrols of Turkish and US soldiers would start soon, Anadolu reported.
Erdogan has repeatedly warned that Ankara would not tolerate further delays to the agreement with the US, which would also stem the flow of refugees into Turkey.
Armed drones and helicopters were already monitoring the region, Erdogan said, a day before he is to meet Russian President Vladimir Putin, the main military backer of the Syrian government.
The two spoke by phone last week after an attack on a Turkish military convoy in north-western Syria, with Erdogan telling Putin that the Syrian government's attacks in Idlib were a serious threat to national security.
Turkey and Moscow reached a deal in September to establish a demilitarized buffer zone in and around Idlib.
The Syrian government, supported by Russian air power, began a military offensive against rebels in Idlib in April.
Syrian forces most recently retook control of Khan Sheikhoun, the largest town in the south of Idlib that is located on an international highway connecting Aleppo and Damascus.
Turkey has 12 observation posts in the region and, despite the advance of Syrian forces in rebel-held areas, said all of its sites would remain in place.
Erdogan said he would discuss the "regime's violations in Idlib" with Putin, CNN Turk reported.
But Russian Foreign Minister Sergei Lavrov said a Syrian military operation in Idlib, involving the town of Khan Sheikhoun, did not violate the Russian-Turkish de-escalation deal.
"The Syrian armed forces with our [Russian] support are not violating any agreement" as "strikes are being delivered against targets that pose a threat," Lavrov said, according to Russian state news agency TASS.
Separately, Akar, the defence minister, said Russia would begin transporting the second battery of the S-400 missile defence system to Turkey on Tuesday.
Turkey started receiving S-400 components on July 12, despite US threats of sanctions. Washington has already suspended Ankara from the F-35 fighter jet programme, saying the missile system is incompatible with NATO and Turkey can't have both.
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