Six South Americans - five Brazilians and one Argentine - have refused to return to Ukraine to play for their football club in the troubled city of Donetsk, risking possible fines and suspensions for breach of contract.
The players remained in France following Ukrainian league champion Shakhtar Donetsk's friendly against Lyon on Saturday.
The Ukrainian military was attempting to retake Donetsk in the east of the country from pro-Russian rebels.
''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 fulfil their contracts could cost the players ''tens of millions of euros'' in compensation payments to the club, he warned.
The players did not return with the team for today's Ukrainian Super Cup against Dynamo Kiev, which would be played in the western city of Lviv.
Akhmetov was Ukraine's richest man and a leading power-broker in the east of the country, where his companies employed 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 would play its home games while the conflict raged. Akhmetov has expressed a preference for the mostly Russian-speaking city of Kharkiv in the northeast.
''We aren't going to take risks and in no way will we take players to somewhere that is dangerous,'' he said.
Donetsk city centre was about 60 kilometres from the crash site of Malaysian Airlines flight MH17, which was shot down last week with 298 people on board.
The six missing players included two of Shakhtar's key Brazilian midfielders, Alex Teixeira and Douglas Costa, as well as Argentine striker Facundo Ferreyra. The others Brazilians were Ismaily, Fred and Dentinho.
''I don't rule out that these players will return to the club soon,'' Akhmetov said, adding that several could arrive today and that the club would not seek to sell them.
Shakhtar coach Mircea Lucescu accused the players' agent, Kia Joorabchian, of being behind their decision to stay in France.
Lucescu told French newspaper L'Equipe that Joorabchian, who was also the agent for Juventus forward Carlos Tevez, convinced the players not to return to Ukraine after telling them they would be able to sign new contracts with other clubs.
''It's a true scandal,'' Lucescu said.
''He took advantage from the situation to abduct them.''
Argentine striker Sebastian Blanco has also refused to return to his Ukrainian club, Metalist Kharkiv, ahead of the new season, according to local media.
Fifa could impose strict penalties in breach of contract cases. Players could be suspended for four months and clubs could face a one-year transfer embargo which prevented them from registering and fielding any new signings.
As Ukrainian champion, Shakhtar has qualified for the group stage of this season's Champions League.
The Ukrainian Premier League season started on Friday. The league has been reduced from 16 teams to 14 after its two Crimean clubs withdrew at the end of last season following Russia's annexation of the peninsula.
Besides Shakhtar, three other teams - Metalurh Donetsk, Olimpik Donetsk and Zorya Luhansk - were based in cities controlled by rebels.
They would use stadiums in other cities because of the conflict.
Another club, Illichivets Mariupol, was based in a city claimed by the self-proclaimed Donetsk People's Republic but under Ukrainian government control.
New Crimean clubs have been created since the Russian takeover, but have yet to be admitted into the Russian league system.
Last week, UEFA announced it would keep Ukrainian and Russian clubs from playing each other ''until further notice'' because of the continuing unrest.
The ruling meant that Russian side Zenit St. Petersburg and Ukrainian club Dnipro Dnipropetrovsk cannot be paired against each other in the Champions League qualifying draw.
But UEFA said the turmoil would not prevent matches from being played at home by Dnipro and Chornomorets Odessa. UEFA also said a security assessment showed that Lviv and Kiev can continue to host matches.
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