ISTANBUL (AP) — U.S. Secretary of State Rex Tillerson is acknowledging severe strains in U.S.-Turkey relations but says he's hopeful of mending ties with the NATO ally and partner in the anti-Islamic State coalition.

Speaking to Turkey-based American diplomats in Istanbul on Monday, Tillerson said he believed the two countries are beginning to restore lost mutual trust. He said since becoming secretary of state he had met three times with Turkey's president and that each time the tone had improved. Tillerson said challenges remain but the first steps to re-establishing relations "on the proper basis" have been taken.

Tensions between Washington and Ankara have been high over U.S. support for Kurdish rebels in Syria, Turkey's crackdown on dissent since last year's failed coup and Turkish allegations the U.S. was sympathetic to the coup-plotters.