Markets in Istanbul tumbled after the U.S. and Turkey stopped issuing visas for each other's citizens in a spat related to last year's failed coup against President Recep Tayyip Erdogan, deepening divisions between NATO members already at odds over the war in Syria.

The Trump administration halted visa services for Turks on Sunday, citing the Oct. 4 arrest of a Turkish citizen employed at the U.S. consulate in Istanbul for alleged involvement in the July 2016 putsch attempt. Erdogan's government responded in kind within hours,

repeating verbatim much of the U.S. statement.

"The implementation of a such a decision by the U.S. ambassador in Ankara is very saddening," Erdogan said at a televised press conference in Kiev, Ukraine. He said that he ordered Turkey's foreign ministry to reciprocate the move. "Turkey is a state of law, not a tribal state," he said.