South African police have stepped up security at Johannesburg Airport after a string of high-profile criminal attacks including an attempted "jumbo-jacking" last week, police said on Tuesday.

Last Thursday's attempt by five armed robbers to seize valuables being unloaded from a KLM flight -- said by local media to include gold and diamonds -- ended in a gunfight that forced the temporary closure of the airport.

The robbers escaped after seriously wounding two police officers.

"The South African Police Service has introduced stringent security measures at Johannesburg International Airport in order to ensure the safety and security of all persons and cargo arriving and departing from our busiest airport," National Police Commissioner Jackie Selebi said in a statement.

He said measures included extra patrols of the airport's perimeter fence and airport buildings.

Members of the police National Intervention Unit and Area Crime Combating Unit would escort valuable cargos to and from planes in armored police vehicles until next January, when specially trained officers would take over the role.

KLM said after Thursday's attack that it was reviewing procedures for the shipment of valuables to Johannesburg.

Local media have reported that an airport restaurant was robbed and a policeman shot while patrolling the airport perimeter in the past three weeks.

KLM suspended transport of valuable cargo from Johannesburg in January 2002 after armed robbers stole 115 million rand (USD$18 million) in cash from one of its planes shortly after it arrived from Amsterdam.

Four airport robberies took place in July and August 2001.

(Reuters)