The European Union's top envoy to Turkey has resigned. It comes at a time of strained relations as Brussels tries to keep afloat a deal to control the flow of refugees into Europe.

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 refugee deal.

The EU is dependent on Turkey to enforce the deal. But the bloc is facing accusations of compromising its values by failing to be critical enough of Turkey's deteriorating record on rights and freedoms. A spokeswoman for the European Commission said Haber would be leaving his role in August.