US Defense Secretary Jim Mattis told his Turkish counterpart that weapons provided to the YPG in Syria would be taken back once Daesh was defeated, Turkish defence ministry sources said on Thursday.
In a letter to Turkey's Defence Minister Fikri Isik, Mattis said the US had taken intensive and determined measures regarding Turkey's security concerns, and would provide a monthly list of the weapons and equipment supplied to the group, recalling that the first inventory list was submitted earlier in June.
Relations between the two NATO allies have become strained due to the support the United States has given the YPG, which Turkey has fought in northern Syria, to support the campaign against Daesh.
The YPG is a leading part of the US-backed Syrian Democratic Forces (SDF) which launched an operation earlier this month to capture Raqqa, Daesh's de facto Syrian capital.
Turkey considers the YPG to be the Syrian branch of the PKK terrorist organisation, which has waged war against the Turkish state since 1984, and has said that it would retaliate against the YPG if it felt threatened by the group.
Ankara fears that the arms provided to the YPG by the US will be used by the PKK.
Although both Turkey and the US consider the PKK a terrorist group, the US claims the YPG has no connection with the PKK.
In his letter, Mattis told Isik that the US would take determined measures to address Turkey's security concerns, the sources said, and that Arabs would comprise 80 percent of the forces to capture Syria's Raqqa from Daesh.
The two leaders will meet face to face in Brussels on July 28 ahead of the NATO defence ministers meetings on July 29.