President Recep Tayyip Erdoğan on Sunday has called German authorities "fascist" for tolerating swastika signs to be drawn on mosques in Germany while reminding trials on neo-Nazi groups targeting the Turkish community have been ongoing for years.

Speaking at an opening ceremony in Istanbul's Gaziosmanpaşa district, the President slammed a German government ban that prevents defense contractor Rheinmetall from shipping some products to Turkey.

Erdoğan said that Germany does not sell weapons to Turkey as a NATO ally, but sells weapons to terror groups.

"They call Turkey's President a 'dictator', but when we call them fascist with good reason, the German authorities get offended," Erdoğan said.

He questioned whether the authorities have taken any actions against neo-Nazi groups who have drawn swastikas on mosques in the country, and stressed that the German authorities have been reckless towards the NSU (Nationalist Socialist Underground) case.

The neo-Nazi gang NSU is accused of racially motivated murders of eight Turks and a Greek in a bombing in a predominantly Turkish neighborhood and a string of robberies between 2000 and 2006.

On the fifth anniversary of the discovery of the murders, a German minister offered apologies for the failure to investigate the murders, while criticism remains for the failure to address links between the state and the gang that were apparently ignored for years.

Turkey-EU relations have been severely strained following a series of bans in Germany, the Netherlands and Austria on Turkish officials' visits for the upcoming referendum campaign in Turkey.

President Erdoğan said on Saturday that Turkey may hold a referendum on whether to continue the European Union accession talks after the April 16 referendum.