ANKARA, Turkey — The U.S. ambassador to Turkey says he hopes the two countries can quickly resolve a spat over the arrest of a U.S. consulate staffer, stressing that cooperation between the two allies helped reduce terror attacks in Turkey.

The U.S. suspended most visa services for Turkish citizens after Turkish authorities arrested a Turkish employee at the American consulate in Istanbul on charges of espionage and alleged ties to U.S.-based cleric Fethullah Gulen, blamed for last year’s failed coup. Turkey retaliated by halting visas services in the U.S.

Ambassador John Bass told reporters the embassy hasn’t received evidence supporting the allegations. Bass said close security cooperation ensured that the Islamic State group was no longer able to carry the kind of attacks that plagued Turkey in 2015 and 2016.

U.S. Secretary of State Rex Tillerson also spoke Wednesday with Turkey’s Foreign Minister Mevlut Cavusoglu about the matter.

Tillerson conveyed his profound concern over the detentions of Turkish employees of the U.S. Embassy and of several American citizens, according to State Department spokeswoman Heather Nauert. He also emphasized the importance of transparency and the need for the evidence behind the accusations to be known, Nauert said.

Ties between the two countries are tense over Gulen, who is wanted in Turkey but who denies allegations of plotting the failed coup. Turkey’s crackdown against Gulen’s movement has led to the arrests of more than 50,000 people for alleged links to terror groups.