Turkish President Recep Tayyip Erdogan said on Friday that Syria is trying to conduct a "psychological operation" in Manbij by sending its troops to the northern Syrian city.

"I contacted my friends in the intelligence units about an hour ago. There is nothing certain yet," Erdogan said in Istanbul of the latest news about Syrian troops entering Manbij.

Earlier in the day, the Syrian army declared that its troops had entered the city and raised the Syrian flag there, following the withdrawal of the Kurdish militia known as the People's Protection Units (YPG).

Ankara decided last week to postpone its planned cross-border military operation against the YPG east of the Euphrates River, after the White House announced its plan to withdraw U.S. troops from Syria.

"Turkey's goal is to teach the YPG a lesson, and we are determined to make it happen," Erdogan said, reiterating his country's resolve to clear the border area of the Kurdish forces.

"We have already taken measures and our armed forces are currently continuing their works in the region," the Turkish leader said, adding that if the Kurdish militia abandons the area, there would be "no other job left" for Turkey.

He also said he might meet with his Russian counterpart Vladimir Putin in Russia in the following days over the developments in Syria.