Authorities on Tuesday warned motorists travelling on the R21 between OR Tambo International Airport and Pretoria to be alert when driving at night.

Several incidents in which robbers have placed rocks in the road have been reported over the past few weeks.

Police confirmed that at least four motorists have been robbed on the R21 highway in the last month.

In all the reported cases, two gunmen held up the occupants of vehicles damaged by rocks and bricks placed on the tarmac.

It is understood several incidents happened on the stretch of road between the Engine 1 Stop and the St George's Hotel between 9pm and 4am.

Police said they were hunting for the two robbers but warned that drivers should remain vigilant.

Anyone with information about the gunmen's whereabouts has been asked to contact Crime Stop.

Anonymous tip-offs can also be sent to Crime Line on 32211.