A crash between an armoured truck and a bus has killed 20 people and injured another 60 in South Africa's northern province of Limpopo, the reginal transport department said on Tuesday.

"Twenty people were tragically killed in an accident, where a cash-in-transit truck lost control and collided head-on with an oncoming bus" on a highway on Monday, the department said in a statement.

Ten of the injured were in serious condition and have been transported to hospital.

Police divers were searching a river that runs along the highway "for people who might have been swept away."

The cause of the accident is under investigation, but heavy rains have been drenching the region in recent days, a spokesman for the department, Tidimalo Chuene, told AFP.

Downpours have caused flooding in several regions of South Africa in recent days, killing at least seven people and leading the government to declare a national disaster.