Turkey summoned Germany's ambassador to its foreign ministry on Monday, Berlin said, amid a mounting row between the two NATO powers.

Turkish diplomatic sources confirmed the summon to Anadolu Agency, but provided no details as to why he was summoned.

Germany's foreign ministry said it was the 17th time its envoy Martin Erdmann had been summoned, underlining the divisions at a time when European powers are counting on Turkey to help contain migrant flows, and to confront Daesh militants over its borders with Iraq and Syria.

According to the sources, the meeting with Erdmann was planned for earlier, but took place only Monday because the ambassador was out of the city.

Foreign Ministry has also summoned Erdmann on Saturday after a mass gathering of supporters and sympathizers of the outlawed PKK terrorist organization in Cologne.

The ministry condemned German authorities for allowing the demonstration, organized in support of the PKK's imprisoned leader, Abdullah Öcalan, and the group's goal for independence in the region.

Ties between Turkey and Germany have been strained since last year, as Turkish leaders slammed Germany for not showing strong solidarity with Ankara against the July 15 coup attempt, and for turning a blind eye to the continued activities of FETÖ in the country.

Ankara has singled out Germany among European countries for embracing some 250 fugitive diplomats and soldiers accused of involvement in the coup with suspected links to FETÖ. A report titled "The FETÖ Settlement in Germany and Germany's FETÖ Policy," says that FETÖ has been using Germany as its main functioning center and that German authorities have embraced the structures of the group with open arms.