KINGSTON, Ont. -- A seven-vehicle crash along a stretch of one of Canada's busiest highways has left four people dead and sent two others to hospital, police said Thursday.

Sgt. Angie Atkinson of the Ontario Provincial Police said the crash took place early Thursday morning on Highway 401, northeast of Kingston, Ont.

Investigators have not yet determined the cause of the crash, which claimed the lives of four unidentified people, she said.

Atkinson said the four victims were all believed to have been travelling in a car involved in the pileup, but police did not have any information on ages, genders or relationships among the group.

The victims' vehicle, which police said caught fire during the incident, was one of two cars caught up in the crash. The collision also involved a commercial vehicle and four transport trucks, Atkinson said.

She said the collision injured two unidentified men.

"One male had to be extricated from the one commercial vehicle and I was advised that his injuries were serious, unknown if they were life-threatening," she said, adding that the other man injured in the crash is also believed to be in serious condition.

The stretch of Highway 401 surrounding the accident just west of Joyceville Road remained closed Thursday morning as police investigated the scene, Atkinson said, adding travellers should account for traffic delays.

"It's quite a lengthy detour that people have to go around, so we advise anybody that has to travel today to leave themselves a little bit of extra time because of the extra traffic that will be on the detour route," she said.

Police said more information will be released as it becomes available.