A government source says at least 10 people are dead and others have been taken to hospital after a highway crash west of Winnipeg.
Authorities have not confirmed numbers, but RCMP are set to hold a news conference later Thursday.
"My heart breaks hearing the news of the tragic accident near Carberry," Manitoba Premier Heather Stefanson posted on social media. "My most sincere condolences go out to all those involved."
Prime Minister Justin Trudeau wrote on Twitter that the news was "incredibly tragic."
"I'm sending my deepest condolences to those who lost loved ones today, and I'm keeping the injured in my thoughts," he said. "I cannot imagine the pain those affected are feeling - but Canadians are here for you."
RCMP are calling the crash "very serious" and are asking people to avoid the area of the Trans-Canada Highway where it intersects with Highway 5.
A damaged semi truck could be seen on the side of the road and a burnt-out van or bus in a ditch along the Trans-Canada.
The pavement was littered with debris - broken glass, a large bumper and what looked like a walker. Seven blue and yellow tarps were stretched out.
Photos supplied to The Canadian Press by a witness from the scene earlier Thursday show what looks like a large van smouldering in a ditch alongside the road.
Nirmesh Vadera, who was working at a business on the side of the highway when the crash happened, said he went outside and saw a transport truck with a smashed engine on the highway.
A multi-passenger vehicle was on fire in the grass on the side of the road. First responders were trying to get people out of the burning vehicle, he said.
"It was burning and all the (firefighters) and medical help and everybody was trying to get them away from the fire. They are just trying to give treatment and everything," he said.
Vadera said the crash looked devastating.
Helicopters with the STARS air ambulance service were sent from Winnipeg and Regina. The crew was made up of doctors, paramedics and nurses.
"We also deployed additional STARS crew to the area on two fixed-wing aircraft, for a total of 14 STARS critical care crew responding in addition to area EMS and other first responders," spokesman Blake Robert said in an email.
Vadera saw one of the helicopters land on the highway and multiple fire trucks, ambulances and police cars arrive.
Health officials say a mass casualty response is underway, but did not give any patient details.
Media outlets have reported a Handi-Transit vehicle was involved in the crash.
Greg Shaw, with Neepawa District Handi Transit, said photos of the scene look just like the 14-passenger buses that his team uses.
He said everyone who drives the buses throughout communities in the area have been shaken.
"We're all just sick," Shaw said."It's just very emotional … We've all made that trip. We've all crossed that highway."
Local MP Larry Maguire wrote on Twitter that he was "beyond saddened."
"My thoughts go out to those mourning the loss of their loved ones and to the emergency responders on the scene," he wrote. "As we always do, let's pull together in love and support for everyone impacted."
Carberry is 170 kilometres west of Winnipeg.