Referees are happy with their performance at the World Cup so far and said a campaign to cut down on foul play and diving had worked but said things could change as the games get more important.

"As a team we are happy, but it's only 25 per cent of the whole way," German referee Markus Merk told reporters on Thursday.

"Now the second phase of the group matches has started and I know with my experience that sometimes it's more difficult."

The matches so far have been cleaner than in past tournaments, with fewer dangerous tackles.

Before the World Cup, soccer's world governing body FIFA said referees would be clamping down on foul play and sent delegations to explain to teams what would not be tolerated.

There would be red cards for elbowing and tackles from behind, players were told, and yellow cards for diving and time-wasting.

The footballers seem to have taken note and the referees have so far largely avoided controversy.

"Usually referees are a topic in the international media when they are bad," said FIFA referee spokesman Andreas Werz.

"I cannot find stories on referees - it's good news."

For the first time at a World Cup, referees have had the option of using microphone and earpiece so they can communicate better with their linesmen, and some say this is helping them to monitor play better.