The EU wants German auto giant Volkswagen to offer customers additional guarantees that repairs carried out to remove its emissions cheating devices will be effective and delivered on time, an official said Monday.

The European Commission had sent VW a letter urging it to do more for European Union consumers unhappy that their American peers are getting a better deal, according to the official, who requested anonymity.

"We need VW to guarantee, in a legally binding way and without any time limit, that the repairs will work and do not have any negative impact," the official said, citing the letter from EU Justice and Consumer Protection Commissioner Vera Jourova.

The letter also called on VW to "take steps toward voluntary compensation" and to look at possibly buying the affected cars back, the official added.

VW admitted in September 2015 that it had installed so-called defeat devices on millions of diesel cars which detected when they were being tested and then drastically reduced emissions so as to comply with regulations.

Brussels has been under fire for dragging its feet on Volkswagen compared to the United States, where authorities not only exposed the wrongdoing, but secured a $16.5-billion (14.8-billion-euro) settlement.

The Commission, the EU's executive arm, complains it currently lacks the authority to fight VW and can only nudge member states to apply EU laws.

Volkswagen has said it will retro-fit affected European cars and argues that paying out even a fraction of what has been agreed for US drivers would quickly bankrupt the company.

"We need to see evidence that this can actually be done. VW needs to convince us that all cars will actually be fixed by autumn 2017," the Commission official said.

Asked about the report, VW told AFP that it welcomed the dialogue with Jourova and was looking forward to discussions "very soon" with her on the issues raised.