A head-on collision between a tour bus and an armoured cash-in-transit van has killed 20 people and wounded 68 others in South Africa.

The vehicles collided on a motorway bridge and the bus then tumbled into the river below.

"Three people were found deceased by the roadside and 16 down by the river," paramedic company ER24 said.

"Most of the fatalities down by the river had been trapped under the bus," it added.

One person died later in hospital.

AFP news agency reports that police divers were also searching the river that runs along the motorway "for people who might have been swept away".

Investigations are under way into what caused Monday evening's crash on the N1 motorway in Makhado in Limpopo province.

This region and others in South Africa have been suffering heavy downpours and flooding, prompting the government to declare a national disaster.

The identities of the victims of the crash have not yet been revealed.