Thirteen people were killed when the cars they were travelling in collided head on on the N2 in Plettenberg Bay on Sunday evening, Western Cape police said.

"After colliding with a VW Polo, an Opel that had been fleeing from traffic officers fell into a ditch and burst into flames," said Captain Malcolm Pojie.

It was not immediately known if the nine people from the Opel burnt to death or if they were already dead when their car burst into flames.

"According to information we received, traffic officers spotted the Opel speeding on the N2 and tried to pull it over. However, the driver continued to drive on and while trying to make a right turn into a gravel road, he collided head-on with the Polo travelling from Port Elizabeth," he said.

The driver of the Polo and two women travelling with him were killed instantly while the third female passenger, reportedly eight-months pregnant, died while being transported to hospital.

ER24 spokesperson Tristan Wadely said most passengers were trapped inside the cars when paramedics arrived at the scene following the 5pm accident.

Although Pojie said there were no survivors, Wadely said one of the people from the accident was taken to the Knysna Provincial hospital in a "serious but stable" condition.

"One person died in an ambulance on the way to hospital and the only survivor of the collision was taken by ER24 ambulance to Knysna Provincial Hospital in a serious but stable condition," he said.

A culpable homicide case was being investigated.