In his letter, Trump called on the Turkish leader to abstain from launching the offensive that would "let the world down" and said that Erdogan should not be a "fool" and should instead work out a "good deal" with the US.
"I returned the letter to him, and there was no reaction", Erdogan told reporters on board a plane while going back from Washington, where he had talks with Trump, as quoted by the NTV broadcaster.
On 9 October, Turkey launched Operation Peace Spring in northeastern Syria to clear the area of Kurdish militants and Daesh* terrorists and create a 30-kilometer safe zone along its the Syrian-Turkish border. Washington and Ankara came to an agreement on 17 October for a 120-hour ceasefire in the area to allow the withdrawal of the Kurdish fighters.
As the five-day ceasefire came to an end, Turkey and Russia reached a deal that would see the Kurdish fighters pull back from the Syrian border area with Turkey. In addition, Moscow and Ankara have begun joint patrols in the operation zone along the Turkish border.
*Daesh (also known as ISIS/ISIL/IS) is a terrorist group outlawed in Russia