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BRUSSELS (Reuters) - The European Union is ready to stick to its migration deal with Turkey but Ankara should not seek to change the rules after they had already been agreed, a top EU official said on Tuesday.

“The EU stands ready to fulfil its part of EU-Turkey deal as long as Turkey agrees to play by the rules, and not with the rules,” European Council President Donald Tusk said on Twitter.

Though much-criticized by rights advocates, the accord helped sharply cut the number of refugees and migrants reaching European shores, giving EU politicians precious breathing space after some 1.3 million people reached the continent last year.

But clouds have now gathered over the deal as Ankara threatens to walk away from it should the EU not ease travel rules for its citizens, a politically contentious discussion in the 28-nation bloc.