The death toll from Pakistan's killer heatwave rose past 1,000 yesterday, with more fatalities expected, as cloud cover and lower temperatures brought some relief to the worst-hit city Karachi.

Mortuaries and gravediggers in Karachi, Pakistan's largest city and economic hub, have struggled to keep up with the flow of bodies since the scorching temperatures began last weekend.

Hospitals have been on a crisis footing and dedicated heatstroke treatment centres have been set up around the city to treat the tens of thousands affected by heatstroke and dehydration.

"The death toll is more than 1,000 and it may reach up to 1,500," Anwar Kazmi, a spokesman for the Edhi Foundation, Pakistan's largest welfare charity and a leading provider of emergency medical care in Karachi, told AFP.

Karachi hospitals have treated nearly 80,000 people for the effects of heatstroke and dehydration, according to medical officials.

After days of temperatures hovering at highs in the mid-40s Celsius (around 110 Fahrenheit), sea breezes and cloud cover have brought some respite to the port city in the last two days. The Met Office forecast temperatures of around 34 degrees Celsius yesterday, with 75 percent cloud cover.

Temperatures of 45 C and higher are not uncommon in parts of inland Pakistan, but Karachi normally remains cooler thanks to its coastal location. This week, however, the cooling breeze that usually blows in off the Arabian Sea was absent.

This year's heatwave has also coincided with the start of the Islamic fasting month of Ramadan, during which millions of devout Pakistanis abstain from food and drink from sunrise to sunset.

Under Pakistani law, it is illegal for Muslims to eat or drink in public during daylight hours in Ramadan, though the crisis prompted some clerics to advise people they should stop fasting if their health is at risk.

The majority of the deaths in Karachi have been among the elderly, the poor and outdoor manual labourers, many of whom are paid by the day and may be reluctant to stop work as it would mean losing income.