A group of German lawmakers has been blocked from visiting troops stationed at Turkey's Incirlik air base, officials said on Monday.
The lawmakers were denied a visit to the base as it was not deemed appropriate at this time, Turkey's Foreign Ministry said.
Some 250 German troops are stationed at Incirlik base as part of the fight against Daesh in neighbouring Syria.
A spokesperson for the German foreign minister said it was "completely unacceptable" for Turkey to keep German lawmakers from visiting their own soldiers.
"A visit by lawmakers must be made possible," Martin Schaefer said, adding that Foreign Minister Sigmar Gabriel would raise the issue with colleagues from other North Atlantic Treaty Organisation (NATO) governments in Washington on Tuesday.
Berlin might consider alternatives
German government spokesman Stefan Seibert said Berlin would consider alternative places to station its soldiers.
Relations between the NATO allies were strained in the run-up to Turkey's April 16 referendum, when Germany banned Turkish politicians from addressing rallies of expatriate Turks. President Recep Tayyip Erdogan accused Berlin of "Nazi-like" tactics.
A narrow majority of voters backed the referendum to change the country from a parliamentary to a presidential system of governance.
Last year Turkey banned German lawmakers from visiting the base for months. The move was a response to a resolution by Germany's parliament to declare the massive deportation of Armenians in the Ottoman era as "genocide," a term Ankara strongly rejects.