The horrific collision took place in the early hours of Friday morning on a rural road near St. Catharines, Ont, said Const. Rich Gadreau of the Niagara Regional Police Service.
Two vehicles were travelling in opposite directions when one of the cars strayed into oncoming traffic, Gadreau said.
The resulting head-on collision caused one of the vehicles to burst into flames.
"One of (the passengers) was able to get out of the vehicle, whereas three others were not," Gadreau said in a telephone interview.
That passenger, plus four occupants of the second vehicle, were airlifted to nearby hospitals, he said.
One man airlifted to Hamilton was believed to be in serious condition, he said, adding the status of the other was still unknown.
Gadreau said the investigation will be complicated by the task of sifting through the burned wreckage.
The names of the victims are not expected to be released for several days, he said.
Jeff Youngblood told a Toronto television station that his father was among those airlifted to hospital.
He said his parents were travelling with another couple as they returned from the Hamilton Airport after a trip to Calgary.
Youngblood said his mother and her friends have already been treated and released.
Niagara Police Chief Jeff McGuire said the crash will have a devastating impact on more than just those close to the victims.
"A number of families have been shattered today and the community will certainly take an impact from this as well," McGuire said.