Witsel underwent an operation on his achilles tendon in January and resumed running just a few days ago, leaving his availability for the June 11-July 11 tournament in doubt.

"With a squad extended to 26 (instead of 23), I can take this risk (on Witsel), he's a unique player," said Martinez.

Fellaini, who has not played for Belgium since the 2018 World Cup after which he left for China, has not been summoned from international retirement.

The squad, which also includes a stand-by list of 11 players, must be officially declared to UEFA by June 1 at the latest.

Belgium have been drawn in Pool B and will play Russia on June 12, Denmark five days later and Finland on June 21.