A woman and three children were killed Thursday after the vehicle they were in drove off a Saskatchewan highway and overturned in a water-filled ditch.

RCMP Sgt. Tammy Patterson said the woman and five children — two of them hers and three from other families  — were travelling along Highway 41 near Melfort southeast of Prince Albert when, "for unknown reasons," the vehicle went off the road.

The vehicle flipped over and was submerged in the water in the ditch.

Patterson said two passersby stopped at the accident scene and tried to help.

"One stopped and actually went into the dugout and pulled one of the victims out of the car and brought him to shore," she said.

Patterson said emergency crews needed a winch to pull the vehicle closer to get the other victims out of the car.

The woman, one of her children and two other girls died at the scene. The two surviving girls were airlifted to a hospital in Saskatoon with undetermined injuries.

The children are all elementary school age and are from St. Brieux, a community of about 500 located approximately 180 kilometres northeast of Saskatoon.

"We can't really determine what the cause of the collision is at this time," Patterson said. "Our reconstructionist is on the scene and he's gathering all the information that he can, so that we can try and make a determination as to what happened."

The weather and road conditions were good at the time of the crash, she said.

With files from the Canadian Press