According to a Turkish translation of the letter supplied to journalists by the National Defense Ministry, Mattis told Fikri Isik the U.S. had taken measures to address Turkey’s concerns.
Ankara views the PKK/PYD, which is the Syrian branch of the PKK, as a terrorist group and fears weapons supplied to the PKK/PYD for the operation to oust Daesh from Syria's Raqqah will end up in the hands of PKK terrorists fighting Turkish security forces.
The PKK/PYD is part of the Syrian Democratic Forces (SDF) involved in the plan to capture Raqqah and who have been supplied with U.S. arms.
In the letter, Mattis pledged the U.S. would provide a monthly list of weapons and equipment supplied to the PKK/PYD. Such a list was handed over to Turkey earlier this month, he said.
He also described the ties between the U.S. and the SDF, which Washington views as its main ally on the ground in Syria, as a tactical partnership and reiterated comments apparently made previously to Isik about the U.S.’s wish to maintain transparency.
It said the U.S.-Turkey relationship was more strategic in nature and went beyond the defeat of Daesh.
Mattis, a retired four-star marine general, said the U.S. had decided to supply the SDF with arms and equipment to take the eastern Syrian city and pointed out that 80 percent of the SDF troops that would go into Raqqah would be Arabs rather than the Kurds of the PKK/PYD.
The PKK, designated a terrorist organization by Turkey, the U.S. and EU, has waged a terror campaign against Turkey since 1984, resulting in more than 40,000 deaths.
Over 1,200 people, including security forces and civilians, have lost their lives since the PKK resumed its decades-old armed campaign in July 2015.