President Tayyip Erdogan made clear on Monday that Turkey would not allow a “safe zone” that it is considering setting up in northern Syria to become a base for Kurdish separatists.

Last week, following U.S. President Donald Trump’s decision on Dec. 19 to withdraw all 2,000 U.S. troops from Syria, Erdogan said he and Trump had discussed Turkey setting up a 20-mile-deep safe zone inside Syria along the length of their border.

Erdogan said Turkey would work with anyone willing to provide it with logistics support for the zone, but that it would take action in Syria if promises were not kept.

“We will never allow a safe zone that will turn into a new swamp for Turkey like the one in Northern Iraq, where we still experience problems,” Erdogan said.

“We are not talking of a safe zone (as protection) against Turkey, but rather one against terrorists.”

Turkish planes regularly attack bases in northern Iraq belonging to the outlawed Kurdistan Workers’ Party (PKK), which has been fighting for three decades for autonomy for southeast Turkey. It is considered a terrorist organization by Turkey, the United States and European Union.

On Sunday, Erdogan told Trump that Turkey was ready to take over security in Syria’s Manbij, where two U.S. soldiers and two American civilian staff died last week in an attack claimed by the jihadist group Islamic State.

Turkey has welcomed Trump’s decision to pull out U.S. forces as it makes preparations for an offensive in northern Syria against the Syrian Kurdish YPG militia, which Turkey says is linked to the PKK.

U.S. forces have been helping the YPG to drive Islamic State out of swathes of northern Syria, and Trump’s decision, which confounded his own national security team, has prompted accusations at home that he is abandoning an ally.

U.S.-Turkish ties have been strained for months over U.S. support for the YPG and a host of other issues.

“If the promises made to us are kept and the process continues, all the better. If not, we have already largely completed our preparations, so we will start taking steps in line with our own strategy,” Erdogan said.

The Syrian Democratic Forces (SDF), a militia allied to the YPG, said last week it was ready to help create the planned safe zone.