South African police launched an investigation on Wednesday into an armed robbery at the main international airport in the city of Johannesburg.

Officials said the heist took place on Tuesday evening at O R Tambo International Airport and that it was unknown what, if anything, was taken.

Some police officials, however, have said the robbers stole an undisclosed amount of money.

Authorities were investigating the alleged use of a vehicle with police markings in the robbery, according to Jacaranda FM, a local radio station.

"That is a subject of police investigation, to determine whether or not the car used belongs to the police or not," police Colonel Athlenda Mathe said in a radio interview.

The Democratic Alliance, South Africa's main opposition party, said it would ask a police watchdog agency to investigate whether police "members or assets" were used in the heist.

The episode "could point to the involvement of corrupt cops," the party said in a statement. It also said it would question the police and transport ministers about security at South African airports.

A company that tries to counter organised crime in the banking industry said it was deeply concerned by the boldness of the criminals as well as their ability to gain access to a secure area.

"It is evident that the heist was executed using accurate information," the South African Banking Risk Information Centre said in a statement.

"This incident exacerbates an already dire situation of attacks on cash in transit vehicles and staff."

Johannesburg's central airport is no stranger to large-scale heists.

According to local reports, in 2006 an estimated $7.67m was stolen from an international aircraft at the same airport.

A similar incident took place in 2014, when armed men took over a vehicle transporting millions of dollars in cash - US dollars, euros and pounds.