BERLIN _ Harry Kewell could miss Australia's crucial World Cup match against Croatia if he is suspended for an outburst against a referee following a 2-0 loss to Brazil.

Kewell will have to explain his post-match comments to a FIFA disciplinary panel after German referee Markus Merk complained that the Australian winger had insulted him.

It was in the "referee's report that (Kewell) insulted the referee. It's been taken up by the disciplinary committee," FIFA communications director Markus Siegler said Monday. "He must now reply by June 20."

Kewell played the entire match in the opening 3-1 win over Japan _ Australia's first ever win at the World Cup _ and went on as a second-half substitute against Brazil on Sunday when the score was 1-0.

The Liverpool forward was clearly upset by a number of calls that went against Australia. Merk called Australia for 25 fouls against only nine for Brazil.

Asked about the exchange with the referee, Kewell told reporters after the game "that's just heat of the moment stuff," adding that he did not think the matter was serious.

Australia plays Croatia in Stuttgart on Thursday and needs at least a draw to advance to the next round. This is the first case of a post-match incident the disciplinary is handling at this World Cup. Sanctions range from a caution to a suspension.

Siegler said the disciplinary committee was still investigating the Italy vs. United States match, in which three red cards were issued. A decision will be made by Friday at the latest.