The European Union officials said Tuesday the EU’s top envoy to Turkey resigned less than a year after he was appointed to Ankara over issues “having to do with Turkey” and not for personal reasons.

Ambassador Hansjörg Haber, a German, was appointed to Ankara on Aug. 31, 2015. He will leave his post on Aug. 1 this year and Brussels will “swiftly appoint” a new ambassador to Turkey, Maja Kocijancic, a spokeswoman for EU foreign policy chief Federica Mogherini, reportedly said.

The move was a further sign of strained ties as Brussels tries to keep on track a landmark deal between the EU and Turkey credited with curbing the flow of refugees and migrants to Europe.

As part of the deal, Ankara will gain visa waivers for its citizens to travel in Europe in exchange for stemming the flow of people making the journey to Greece by boat. Brussels also promised Turkey to speed up its long-dormant EU accession talks as Europe deals with its worst refugee crisis since World War II.

The resignation comes a month after Haber was summoned by Turkey's foreign ministry over comments critical of President Recep Tayyip Erdoğan and Ankara's handling of the refugee accord.

Ömer Çelik, Turkey's EU affairs minister, told reporters it had become impossible for Haber to function in his role. “The first rule for all diplomats is to respect national values, which include the common value of the office of the presidency,” Çelik said.

“The comments made by the ambassador in question violated this fundamental rule and the necessary reaction was made. Following that, I believe his work no longer had a function or meaning,” he added.

EU officials, however, declined to comment on the reason behind Haber's resignation.

Turkey was Haber's first assignment as a German diplomat between 1993 and 1996, and he returned as the European Union ambassador to Turkey in September 2015.