Fifa president Sepp Blatter has criticised English clubs Chelsea and Coventry for their handling of international players.
Blatter was angered by Chelsea and Coventry
Adrian Mutu complained after being told by Chelsea boss Jose Mourinho he was not fit enough to play for Romania.
And Stern John was warned he might lose his place in the Coventry side if he joined up with Trinidad and Tobago.
But Blatter described both incidents as "unacceptable" and said: "We will write to the clubs and ask for fair play."
Fifa reacted after Mutu went public in revealing his "open conflict" with Chelsea coach Mourinho.
Despite his coach's insistence that he was not ready for international football, the striker has declared himself fit and travelled to the Czech Republic for Saturday's World Cup qualifying match.
Coventry have also angered football's world governing body by warning John he might not get back in the Sky Blues squad if he travels to play for Trinidad and Tobago.
"Mutu is in an open conflict with his coach. Mourinho has
forbidden him to play for Romania, saying he is injured, and Mutu says this is
not true," Blatter said.
"Stern John has been asked to play for T&T and has been threatened by his
coach Peter Reid that if he goes to play he may lose his position on the team.
"We cannot accept this happening at a time we are trying to find a workable situation with the international calendar."