European Union auditors are struggling to work out whether some of the billions the bloc has sent Turkey to help it cope with Syrian refugees is having an impact.

In their report, the EU auditors raised concerns about the ability of non-governmental organisations (NGOs) to operate projects, as envisaged under the 6 billion euro ($6.4 billion) deal, given Turkey's authoritarian turn since a failed coup in 2016 and its crackdown on dissent.

European Union auditors have said they are unable to determine the impact of the billions of euros the bloc gave to Turkey to help it cope with migrants crossing in from Syria.