The European Union’s top envoy to Turkey has resigned, less than a year into the job, EU officials said on Tuesday, in a further sign of strained relations as Brussels tries to keep a deal credited with curbing the flow of illegal migrants to Europe on track.

Hansjoerg Haber’s resignation comes a month after he was summoned by Turkey’s foreign ministry over comments critical of Ankara’s handling of the migration accord. An EU official said his departure was “not for personal reasons.”

His office said Haber, a German career diplomat, was leaving his post, without giving any reason.

The resignation comes at a sensitive time in EU-Turkish relations as the bloc recently brokered a deal with Turkey to curb illegal migration.

The agreement called for irregular migrants who arrived in the Greek islands from Turkey after March 20 to be sent back to Turkey.

The EU, in turn, is to resettle one Syrian refugee from Turkey to the bloc for each Syrian that Greece returns to Turkey. Turkey also stands to receive up to 6 billion euros ($6.71 billion), visa-free travel and fast track negotiations on EU accession.

Visa-free travel to Europe, however, is conditioned on Turkey amending its anti-terrorism laws among other criteria.

The EU says Turkey must narrow its definition of terrorist crimes, which leads to extensive application of the law against intellectuals, Kurdish sympathizers and critics of President Tayyip Erdogan, including journalists and academics.

That requirement has proved a sticking point and the subject of sharp exchanges between EU and Turkish officials. Ankara argues it cannot narrow its laws when it faces the twin threats of ISIS and Kurdish rebels.