U.S. National Security Adviser H.R. McMaster held talks on Sunday in Istanbul with Turkish President Tayyip Erdogan's spokesman, a presidential source said, amid persistent tensions between the NATO allies over Syria and other issues.

Turkey is targeting the Syrian Kurdish militia YPG in a weeks-long air and ground operation in Syria's Afrin region. Turkey considers the YPG a terrorist group, but it is a key component of U.S.-backed forces battling Islamic State (IS) militants in Syria.

Erdogan has promised to sweep the militia from Turkey's southern border, a move that could put Turkish troops in confrontation with U.S. forces on the ground in Syria.

McMaster's talks with Erdogan's spokesman Ibrahim Kalin in Istanbul come days ahead of an expected visit to Turkey by U.S. Secretary of State Rex Tillerson.

"In the meeting (of McMaster and Kalin) the long-term strategic partnerships between Turkey and the U.S. were confirmed and the sensitivities and priorities of the two countries were handled," the presidential source said.

The two men evaluated regional developments, factors that have damaged relations between the allies and developing ways of combating terrorism, the source said, but provided no further details.

Last week Turkish Foreign Minister Mevlut Cavusoglu said he would tell Tillerson during his visit that the two countries must repair damaged mutual trust. No dates have yet been confirmed for the Tillerson visit.

Relations have also been strained by Washington's failure to extradite a U.S.-based Muslim cleric blamed by Turkey for a failed coup attempt against Erdogan in 2016. Washington says Ankara has not provided enough evidence for the extradition.