Belgium coach Roberto Martinez left AS Roma midfielder Radja Nainggolan out of his squad for next month's World Cup in Russia, insisting on Monday it was a purely tactical move and not due to past clashes over discipline.

The Spaniard sprung a surprise by not naming his final 23-man squad as expected, instead listing 28 players who will be whittled down after a friendly against Portugal on June 2.

"Radja is a top player," Martinez told a news conference, acknowledging Nainggolan's key role in Roma's run to the Champions League semi-finals, but saying he could not offer him a similar place in a Belgian side he described as more offensive.

Coach and player had an "honest" discussion after Martinez flew to Rome on Sunday to break the news.

There was no doubt about the popularity of Nainggolan among Belgian fans following his key role at Euro 2016, the manager said, but a coach must be ready to take hard decisions for the good of the team

"I don't think Radja is a player to be used in a small role," Martinez said when asked about Nainggolan's reported willingness to go with the squad to Russia "even as a goalkeeper".

"The reason that Radja's not in the squad is purely a tactical decision," Martinez told reporters.

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