German Chancellor Angela Merkel and top EU officials are due to visit the Turkish-Syrian border to promote a controversial month-old migrant deal.

The visit comes amid questions over the legality of the EU-Turkey pact, which deports back to Turkey migrants who do not qualify for asylum in Greece.

Human rights groups say Turkey is not a safe place to return people.

Turkish officials have warned the deal could collapse if demands for visa-free EU travel for its citizens are not met.

The agreement says Turkey must meet 72 conditions by 4 May to earn the visa waiver, but diplomats say only half of those points have been met so far.