WASHINGTON, Feb. 8 (Xinhua) -- U.S. Secretary of State Rex Tillerson will kick off a nearly week-long tour on Sunday to Turkey and four other Middle East countries amid mounting tensions between Washington and Ankara over Turkey's military offensive in Syria, the U.S. State Department said Thursday.

In Turkey, Tillerson will meet with senior Turkish officials to discuss "a range of bilateral and regional issues," said the U.S. State Department Spokesperson Heather Nauert in a statement.

Tension has been escalating between the two NATO allies amid Turkey's ongoing military operation into the Syria's Afrin against the People's Protection Units (YPG), which Ankara considers as the Syrian affiliates of the Kurdistan Workers' Party (PKK), listed by Turkey, the European Union and the United States as a terrorist organization.

Ankara's operation followed U.S. calls to establish a Kurdish-led "border security force (BSF)" in the future in Syria along the 900-km border with Turkey, a move that has angered Turkey.

The BSF would be mainly constituted by the YPG, which has been supported politically and militarily by the United States since 2014 in the battle against the Islamic State.

According to the statement, apart from Turkey, the top U.S. diplomat will also travel to Jordan, Lebanon, Egypt and Kuwait on Feb. 11-16 and engage with leaders of the countries on topics including ongoing crisis in Syria.