Shakhtar Donetsk's owner says six players have refused to return to Ukraine for the new season as conflict rages around the club's home city.
Five Brazilian and one Argentine player remained in France following a friendly against Olympique Lyon on Saturday.
In a statement on the club's website, owner Rinat Akhmetov warned the players that "in the first instance they will suffer" if they do not return, because their actions could activate compensation clauses forcing them to pay "tens of millions of euros" to Shakhtar.
The Ukrainian military is attempting to retake the city of Donetsk in the east of the country, which is controlled by pro-Russian rebels.
The Ukrainian Football Federation has yet to rule where Shakhtar will play its home games while the conflict rages.