Turkish President Recep Tayyip Erdogan said Wednesday he returned a letter by his U.S. counterpart that threatened Ankara over its anti-terror operation in northeastern Syria and bemoaned Washington's treatment of the YPG/PKK terror group's leader, Eurasia Diary reports citing Anadolu Agency.

"I presented the letter to Mr. President and especially felt sorrow that a U.S. president took a terrorist called Ferhat Abdi Sahin as an addressee," said Erdogan during a press conference with Donald Trump in Washington.

Sahin, also known as Mazloum Kobani, is the ringleader of the PYD/YPG, the Syrian offshoot of the PKK terror organization. Trump repeatedly praised the terrorist leader and said he looks forward to seeing him.

Erdogan said Sahin has caused the deaths of hundreds of Turkish people and is the adopted son of Abdullah Ocalan, the jailed leader of the PKK terrorist organization.

"That the U.S., which we call our strategic partner, welcomes such a person actually saddens us. Likewise, this person is welcomed by Russia. It is hard to comprehend this within the scope of the fight against terrorism across the world," said Erdogan.