European Union countries Croatia, Romania and Bulgaria all meet the requirements to become full members of the bloc's passport-free Schengen area, German Chancellor Olaf Scholz said on Monday.

"Schengen is one of the greatest achievements of the European Union, and we should protect and develop it. This means, incidentally, closing the remaining gaps," he said, according to the text of a speech for delivery in Prague.

"Croatia, Romania and Bulgaria fulfil all the technical requirements for full membership. I will work to see them become full members." (Reporting by Sabine Siebold, Robert Muller, Jason Hovet Editing by Paul Carrel)