"It is with regret and disappointment that, following Wednesday's Group B game between Portugal and Morocco, FIFA has learned of reports in the media concerning referee Mark Geiger. It has been claimed that Mr. Geiger asked for the shirt of the captain of Portugal during the half-time interval. Mr. Geiger strongly refutes these claims and categorically states such a request was not made," FIFA said in the statement.
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FIFA also called on the teams to respect all principles of fair play.
"FIFA unequivocally condemns the allegations supposedly made by a member of the Moroccan team. FIFA referees are under clear instructions with regard to their behavior and relationship with the teams at the World Cup and it can be confirmed that Mr. Geiger has acted in an exemplary and professional manner as an appointed match official," world football's governing body added.
The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the matches being played in 11 cities across Russia.