Turkish President Tayyip Erdogan has told visiting U.S. Secretary of Defense James Mattis that Turkey felt unease over American backing for Kurdish militant group YPG (the Kurdish People's Protection Units), viewed by Ankara as a terrorist group.

Mattis arrived for the one-day visit after stopping in Iraq to review progress in the campaign against the Islamic State group.

He met with Erdogan in Ankara after talks with Defense Minister Nurettin Canikli. Turkish intelligence chief Hakan Fidan also took part in the meeting with Erdogan.

Turkey, an important NATO ally of the United States and part of the coalition fighting the IS militants, is irritated that Washington has been arming the YPG in the assault on the militants' stronghold of Raqqa, in northern Syria.

Ankara says the YPG is the Syrian affiliate of the outlawed Kurdistan Workers' Party (PKK), but Washington has been impressed with its ability to combat IS on the ground.

During the meeting, Erdogan told Mattis that Turkey was "uneasy over the U.S. support" for the YPG, presidential sources told journalists.

The Kurdish regional government in northern Iraq -- whose peshmerga security forces are playing a key role in the fight against IS -- is planning its own independence referendum next month.

Mattis met August 22 with Iraqi Kurdish leader Massud Barzani in Erbil to express U.S. opposition to the referendum.

Erdogan and Mattis reaffirmed their opposition to the vote, the Turkish presidential sources said August 23.

Based on reporting by Reuters and AFP