Police killed eighteen people in a shootout in South Africa's northeast on Friday, saying they had targeted suspects who were planning to rob a cash truck.

"As soon as police approached the address, the group of suspects began shooting, police retaliated," the South African Police Service (SAPS) said in a statement.

"Sixteen males and two females were declared dead at the scene."

A police officer was "injured very seriously" and taken for medical care, SAPS National Commissioner General Fannie Masemola told reporters in Makhado, Limpopo province, around 400 kilometres (250 miles) northeast of Johannesburg.

The shootout lasted for about 90 minutes, said Masemola.

According to police, who had been tracking the movements of the suspects for a few days, a probe began in January with the aid of the country's intelligence unit.

"Our intention was to arrest them before they go out and commit the crime" Masemola said.

Cash truck heists happen regularly in the crime-ridden southern African country.

Last year 10 suspects died during a cash-in-transit shootout with police after the would-be robbers fired at a police helicopter and wounded one of the pilots.

In May police minister Bheki Cele said there was a "stubborn increase" in cash-in-transit robberies of more than 20 percent, with 64 cases reported during the first three months of the year.

The crime statistics, which are presented by the minister every quarter in a live broadcast, have been steadily rising over the past few months in a country that has one of the highest crime rates in the world.

Police have said they are "clamping down on serious and violent crimes throughout the country".

Four people who were found in "another safe house" were arrested in connection to the alleged cash truck robbery plan.

Masemola said there was also an "explosive that was ready to be used because they wanted to hit one of the cash depots in the province".

He said the suspects planned to use an ambulance to transport the money to the country's Gauteng province.