"There is a heightened risk that extremist groups will conduct terrorist acts within South Africa in the near future," the US state department said.

The warning came as US President Barack Obama wished the American World Cup football team good luck.

"I just want to say how incredibly proud we are of the team," said Obama, who was joined by former President Bill Clinton to give the players a presidential send-off at the White House in Washington in a BBC report.

"Everybody's going to be rooting for you," he said.

"And although sometimes we don't remember it here in the United States, this is going to be the biggest world stage there is."

In a statement, the state department said it had no information on any specific, credible threat during the tournament, but noted that such threats have been reported in the media.

South Africa has mobilised thousands of specially trained police to deal with fans' safety.
Some 350,000 people are expected to visit South Africa for the World Cup, which is being held in Africa for the first time and starts on 11 June.