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.
"If they don't come then in the first instance they will suffer," the club's billionaire owner Rinat Akhmetov said in a statement on the Shakhtar website.
Refusing to fulfill their contracts could cost the players "tens of millions of euros" in compensation payments to the club, he warned.
The six players include two of Shakhtar's key midfielders, Alex Teixeira and Douglas Costa, as well as Argentinian striker Facundo Ferreyra.
"I don't rule out that these players will return to the club soon," Akhmetov said, adding that several of them could arrive Monday and that the club would not seek to sell them.
The Ukrainian military is attempting to retake the city of Donetsk in the east of the country, which is controlled by pro-Russian rebels.
Akhmetov is Ukraine's richest man and a leading power-broker in the east of the country, where his companies employ hundreds of thousands of people.
He has publicly opposed the rebels and earlier this month pleaded with Ukrainian forces not to bomb Donetsk.
The Ukrainian Football Federation has yet to rule where Shakhtar will play its home games while the conflict rages.
Akhmetov has expressed a preference for the mostly Russian-speaking city of Kharkiv in the north-east.
"We aren't going to take risks and in no way will we take players to somewhere that is dangerous," Akhmetov said.
Argentinian striker Sebastian Blanco has also refused to return to his Ukrainian club, Metalist Kharkiv, ahead of the new season, local media report.
Shakhtar's first game of the season is the Ukrainian Super Cup against Dynamo Kyiv, which will be played in the western city of Lviv.
As Ukrainian champion, Shakhtar has qualified for the group stage of this season's Champions League.