FIFA secretary general Jerome Valcke has expressed concern over match fixing in international football.
In an interview with Brazilian television channel Sportv, Valcke said there was no way of knowing if games played during the World Cup had been manipulated, reports Xinhua.
"It exists and it's something that we are not going to eliminate hundred percent," the governing body's World Cup chief said.
"It happens at the highest level and at the lower levels. What we can do is control legal betting. We work with two companies that monitor this around the world. But it's very difficult to fight against what is done behind the scenes."
Valcke's comments came after German magazine Spiegel claimed the Group A match between Cameroon and Croatia June 18 was fixed.
The magazine alleged that Cameroon agreed with a betting ring before the match to lose 4-0.
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Days later FIFA revealed the clash between Cameroon and Brazil June 23 was at risk of being rigged.
Media reports said alarm bells rang due to Cameroon's prior elimination from the tournament and reports of conflict within the Indomitable Lions' team.