Six more people have died in Montreal due to a heat wave, bringing to 12 the city's total death toll from the extreme weather conditions that have gripped central and eastern Canada, health officials said on Wednesday.

Montreal previously raised the city's response level to "intervention" from "alert" after a spike in heat-related calls to the government's health information line and for ambulances.

David Kaiser, a physician at Montreal's Public Health Department, said most of the victims lived alone and none had access to air conditioning.

Separately, five people died in Quebec's Eastern Townships, which health officials said could be linked to heat.

Canadian Weather issued a heat warning for southern Quebec, which includes Montreal. The advisory was expected to be in place until Thursday. Heat warnings were also issued for much of Ontario, Nova Scotia and New Brunswick.

Montreal reached a high of 34 degrees Celsius (93 degrees Fahrenheit) on Wednesday, a peak that is expected to continue on Thursday, before cooling to 24 degrees Celsius on Friday.

According to Canadian Weather's advisory, humidex values in southern Quebec will reach near 40 on Wednesday, but conditions will grow "even more uncomfortable" on Thursday, when humidex values reach 43. Humidex, or the humidity index, expresses how temperatures feel when incorporating the effect of humidity.

In a statement issued on Wednesday, Montreal's emergency services reported over 1,200 calls per day related to the heat, which represents 30 percent more than its busiest days. Montrealers were urged not to dial 911 for non-urgent ailments, as the influx of calls causes delays in interventions for urgent matters.