"We are taking another step in establishing a 30-kilometre security zone along our southern border. We will clean up Tell Rifat and Manbij", he said, referring to two northern Syrian cities.

Erdogan said they would then proceed, "step by step, into other regions".

For a week now, Turkey's leader has been threatening to launch an operation against fighters of the Kurdistan Workers' Party (PKK).

He is also targetting the People's Protection Units (YPG), a Syrian-Kurdish group it considers to be part of the PKK.

"We will see who supports the legitimate security operations carried out by Turkey and who tries to oppose them," said Erdogan.

He warned over the weekend that Turkey would not wait for permission from the United States before launching such an operation.

Washington last week warned Turkey against launching a military operation into northern Syria, saying it would undermine regional stability and put US forces serving there at risk.

Erdogan on Tuesday told Russian President Vladimir Putin that a 2019 agreement signed between the two countries allowed for the creation of a security zone along the Turko-Syrian border.

He has also opposed the recent applications of Finland and Sweden for NATO membership, over what it considers their leniency toward Kurdish militant groups.