Johannesburg - The government has issued a stern warning over unruly behaviour during the Soccer World Cup and promised a clampdown on illegal ticket sales.

A stampede by fans during a warm-up match between Nigeria and North Korea on Sunday left 15 people injured.

"The government will not tolerate any unruly, disruptive and unsafe behaviour that impacts negatively on those attending public events," government spokesman Themba Maseko said in a statement on Wednesday.

Maseko urged fans not to enter stadiums during the World Cup without valid tickets.

"Those found in possession of illegal tickets will be prosecuted," he said.

The government has taken pains to assure foreign visitors that they will be safe during the month-long tournament, which starts on Friday.

Security has been one of the biggest issues ahead of the World Cup because of South Africa's high rate of violent crime - with 50 murders a day, almost the same rate as the United States which has six times the population.

The country's crime rate is among factors blamed for lower-than-expected foreign bookings to attend the World Cup.

The government and local organisers have insisted fans will be protected by a $174 million security plan including more than 40 000 specially deployed police.