A commercial tour bus carrying 21 people has crashed in Cape Breton on Sunday, seriously injuring one person while nine others suffered moderate injuries.

A tour bus rollover on the Cabot Trail in Nova Scotia on Sunday left at least 10 people injured. (George Mortimer/CBC)

RCMP Sgt. Brigdit Leger said the bus was about three kilometres south of Neils Harbour, on the Cabot Trail, when it rolled over.

"It appears as though the driver lost control of the bus and the bus ended up rolling into the ditch on its side. However, our investigation is ongoing to determine why the driver lost control of the bus," she said.

Leger said one passenger was seriously injured and nine others suffered moderate injuries.

RCMP got the 911 call for help just before 2:30 p.m.

A LifeFlight aircraft and a number of other emergency vehicles were sent to the area.

Lynn Gilbert, a spokeswoman for the Cape Breton Regional Health Authority, said four people with "undetermined injuries" were taken by ambulance to the regional hospital in Sydney.

The nature and extent of their injuries will be assessed when they reach Cape Breton Regional Hospital.

It's unclear if the person with serious injuries is in this group.

A long lineup of vehicles wait as a medical evacuation helicopter prepares to land near the scene of a rollover on the Cabot Trail in Nova Scotia. (George Mortimer/CBC)

Gilbert said six others whose injuries were not life-threatening were taken to the local health centre in Neils Harbour.

The Cabot Trail is closed. One lane is expected to reopen by about 6:30 p.m. local time.

Johnny Buchanan, the county councillor for the area, said that section of the road is in good condition, with no significant curves.

No other details are known about the bus or passengers.