Authorities earlier reported 323 deaths in only Karachi in the three-day heat disaster.
But more deaths were reported by officials in the Sindh Province, said National Disaster Management Authority spokesman Ahmed Kamal.
Officials also told CNN that the number of patients treated for heat stroke in Jinnah hospital, the largest in the Karachi, is 2,360. Karachi, a seaside city, is Pakistan's largest.
At least one city morgue, CNN affiliate Geo.tv
has reported, has been overwhelmed with the numbers of dead.
Death tolls in local media reports vary.
The Sindh provincial government has declared a state of emergency in all its government hospitals, canceling leave of medical personnel and bringing in further medical supplies.
The record-breaking temperatures would likely bring more deaths before cooler weather, forecast for later in the week, arrive.
Saturday's temperature reached 44.8 degrees Celsius (112.64 degrees Fahrenheit) -- the highest-recorded temperature in Pakistan in the past 15 years.
Sunday's temperature dipped slightly to 42.5 Celsius (108.5 F).
Ramadan obligations add pressure
Citizens in this predominantly Muslim country are observing Ramadan, the holy month when Muslim faithful around the world fast from sunup until sundown.
This means, that amidst these scorching temperatures, Pakistanis are foregoing food and water.
Making matters worse, Karachi is dealing with frequent power outages as the electricity grids are unable to keep up with the demand in the city of 20 million.
To the east, a heat wave struck India last month, killing more than 2,000 people