Every match in the tournament is monitored by the Early Warning System that detects irregular betting patterns.
But Marco Villiger, FIFA's Legal Director, told a media briefing it keeps a particularly close eye on high risk games which complete the first round at the finals.
The top two advance from the eight first-round groups of four and by the time teams play their final group match some have already clinched a second-round place while others know they have no chance of qualifying for the knockout stages.
"These are the high risk matches from a betting perspective," said Villiger.
"Matchfixing is the biggest threat facing football at the moment and there is a big risk in the last group matches, especially if a team is involved which has already qualified, or is already out.
"We use our all our means to focus on those who have already qualified or are out. The question is, are these high risk matches and the answer is yes, they are."
FIFA informed every national association, player, coach, delegation member and referee before the start of the tournament they would watch out for irregular or illegal betting patterns.
"I would not believe the World Cup could be a target for the betting mafia but in the meantime we have to be prepared that even the World Cup could be, not just lower league matches."
HOTLINE
A telephone hotline has been set up so that anyone involved in a match - players, coaches or referees - approached by a third party offering them money or a bribe for betting or match-fixing can inform FIFA in confidence immediately.
"The threat is here," said Villiger, "but we're working closely with our colleagues at (European football's governing body) UEFA, with Interpol, with 400 bookmakers around the world, to do all we can to keep the game clean."
He added that live betting - for example when bets are placed on the first throw-in or yellow card at a match - was harder to track. "For this World Cup we are focusing much more on the investigative part of betting.
"We have international cooperation with Interpol and we have a number of informants who provide us with high level information."
So far FIFA has not detected any signs of irregular betting at the finals but it is not resting on its laurels.
"The World Cup, the Euros, the Champions League, other big tournaments are always vulnerable to the betting syndicates. We are binding all our forces together to fight this threat."
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