ANKARA (Reuters) – The United States has told Turkey it will take back weapons supplied to the Kurdish YPG militia in northern Syria after the defeat of Islamic State, Ankara said on Thursday, seeking to address Turkish concerns about arming Kurds on its border.

Turkish defense ministry sources said U.S. Defense Secretary Jim Mattis also promised his Turkish counterpart to provide a monthly list of weapons handed to the YPG, saying the first inventory had already been sent to Ankara.

Turkey sees the YPG as an extension of the outlawed Kurdish PKK, which has been waging an insurgency in the country’s southeast since the mid-1980s. It has said supplies to the YPG have in the past ended up in PKK hands, and described any weapon given to the force as a threat to its security.

The United States sees the YPG as an essential ally in the campaign to defeat Islamic State in Raqqa, the jihadists’ main urban base in Syria. The fight for Raqqa was launched two weeks ago, piling pressure on Islamic State which also faces defeat in its Iraqi stronghold of Mosul.

Mattis told Turkish Defense Minister Fikri Isik that a detailed record of all equipment provided to the YPG was being kept and that all the weapons would be taken back after Islamic State was defeated, the sources said.

In a letter to the minister, Mattis also said that Arab fighters would form 80 percent of the forces which will recapture Raqqa. Once the mainly Sunni Arab city was taken, it would be held by Arab forces, the sources said he told Isik.

Relations between the NATO allies have been strained by President Donald Trump’s decision to arm the YPG, despite protests from President Tayyip Erdogan who set out Turkey’s objections at a White House meeting last month.

Erdogan has said Turkey would retaliate against the YPG if it felt it was threatened by the group.

A Syrian war monitor and Kurdish sources said on Wednesday that Turkey had sent military reinforcements including troops, vehicles and equipment into an area of northern Syria where it has been fighting Islamic State and YPG forces.

Turkish officials have not commented, but the British-based Syrian Observatory for Human Rights said Turkish reinforcements headed south of Azaz town, which is held by Turkish-backed Syrian rebels and is close YPG-controlled territory.

(Reporting by Ece Toksabay and Tuvan Gumrukcu; Editing by Dominic Evans)