In a letter to Turkey's Defence Minister Fikri Isik, Mattis said the United States had informed Turkey about the weapons it had given the YPG and that it would provide monthly lists of the arms supplied, the sources said in a statement.

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 Islamic State.

In his letter, Mattis told Isik that the United States 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 Islamic State.

The YPG is a leading part of the US-backed Syrian Democratic Forces (SDF) which launched an operation earlier this month to capture Raqqa, Islamic State's de facto Syrian capital. Turkey has said that it would retaliate against the YPG if it felt a threat from the group.