The head of Winnipeg's firefighters' union says he's worried another busy weekend could be ahead, as crews are getting worn out by numerous arson and other fires.

Firefighters responded to at least six deliberately set fires every night over the past three weekends, according to the United Firefighters of Winnipeg.

Most of those fires were set in the downtown area, said union president Alex Forrest, who added that the hot weather and high humidity have also been exhausting firefighters.

"Fire crews in the downtown core basically go from one fire to another fire to another fire. So with the heat, we have to give them a longer time to rehab after each and every fire," he told CBC News on Friday.

The City of Winnipeg says there were 22 fires reported over the Canada Day long weekend.

Forrest said almost all of the deliberately set fires are happening downtown, and the overtaxed crews are having to call in firefighters from the suburbs to help.

"Whenever you have a huge number of arsons like we've had these last number of weekends, it creates a domino effect for depleting resources throughout the city," he said.