At least 23 people have been killed and five others injured after a bus carrying patients and healthcare workers collided head-on with a truck in southern Brazil, authorities said.

The crash occurred in the early hours of Wednesday on the BR-373 highway in the state of Parana, near the city of Ipiranga, according to the Federal Highway Police.

Most of those killed were travelling on the bus, which belonged to the municipal health department of Imbituva. The truck driver was also among the dead.

The bus was transporting patients and their companions to hospitals in and around Parana's capital, Curitiba, approximately 180 kilometres (110 miles) away, police said.

Preliminary evidence suggests the truck crossed into the opposite lane before the collision. Investigators are working to determine what caused the vehicle to veer across the highway.

Following the crash, Parana Governor Ratinho Junior declared three days of official mourning across the state.

"It's a very sad day for Parana," Junior said.

The five survivors were taken to hospitals in the nearby city of Ponta Grossa, according to local media reports.

The highway was closed in both directions after the crash. Road operator Via Araucaria said there was "no estimated time" for the road to reopen.

Local authorities have been providing psychological support to the families of those who were travelling on the bus. The identities of the victims had not been released at the time of publication.