The European Commission is to grant Turkey conditional approval this Wednesday for visa-free travel within the EU's Schengen area, sources have told the BBC.

The move is part of a deal in which Turkey has agreed to take back migrants who have crossed the Aegean to Greece.

But Turkey must still meet EU criteria, BBC Europe Editor Katya Adler says.

She says the EU fears that if the visa agreement slides, so will Turkey's commitment to stopping migrants.

The huge numbers of migrants and refugees arriving in Europe from Turkey, and from North Africa, has caused a political crisis among EU states.

Under the EU-Turkey agreement, migrants who have arrived illegally in Greece since 20 March are to be sent back to Turkey if they do not apply for asylum or if their claim is rejected.

For each Syrian migrant returned to Turkey, the EU is to take in another Syrian who has made a legitimate request.

Human rights groups question the deal's legality and argue that Turkey is not a safe place to return people to.

Last month, however, European Council President Donald Tusk said the deal had begun to produce results.

He praised the Turkish government as "the best example in the world on how to treat refugees", despite criticism by rights groups of the agreement.

At the same time, Turkish PM Ahmed Davutoglu said his country had fulfilled its part of the agreement and that the issue of the visa waiver for the EU's Schengen area was "vital" for Turkey.

The agreement says Turkey must meet 72 conditions by 4 May to earn access by the end of June, but diplomats have said that only about half of those points have been met so far.