Pakistan was in the grip of a heatwave on Friday, with parts of the nation already scalded by temperatures of nearly 50 degrees Celsius as officials warned of acute water shortages and a threat to health.

Swathes of Pakistan have been smothered by high temperatures since late April, in extreme weather the World Meteorological Organization (WMO) has warned is consistent with climate change.

On Thursday the city of Jacobabad in Sindh province hit 49.5C (121 degrees Fahrenheit), the Pakistan Meteorological Department (PMD) said, with temperatures forecast to remain the same until the end of the week.

Nationwide the PMD alerted temperatures were between 6C and 9C above normal, with the capital Islamabad, as well as provincial hubs Karachi, Lahore and Peshawar, recording temperatures around 40C on Friday afternoon.

"This year we have jumped from winter right into summer," said PMD chief forecaster Zaheer Ahmad Babar.

Pakistan has endured heightened heatwaves since 2015, he said, focused in upper Sindh province and southern Punjab province.

"The intensity is increasing, and the duration is increasing, and the frequency is increasing," he told AFP.

Punjab province irrigation spokesman Adnan Hassan said the Indus river -- Pakistan's key waterway -- had shrunk by 65 per cent "due to a lack of rains and snow" this year.

Sheep have reportedly died from heatstroke and dehydration in the Cholistan Desert of Punjab, Pakistan's most populous province which also serves as the national breadbasket.

"There is a real danger of a shortfall in food and crop supply this year in the country should the water shortage persist," Hassan said.

On Tuesday climate minister Sherry Rehman warned residents in the eastern megacity of Lahore "to take cover for the hottest hours of the day".

The heatwave has also ravaged India, with temperatures in parts of Rajasthan hitting 48.1C on Thursday.

Pakistan -- home to 220 million -- says it is responsible for less than one per cent of global greenhouse gas emissions.

However it ranks as the nation eighth most affected by extreme weather events, according to a 2021 study by environmental group Germanwatch.

While extreme heat has obvious effects it can also trigger cascading disasters which pummel Pakistan's generally impoverished population.

The mountainous portions of Pakistan are home to more than 7,000 glaciers, a number larger than any region outside the poles.

Quickly melting glaciers can swell lakes which then burst their banks and unleash torrents of ice, rock and water in events known as glacial lake outburst floods.

Last weekend a key highway bridge in the Gilgit-Baltistan region was swept away in flash flooding caused by glacier melt.

In April officials warned there were 33 lakes in Pakistan in danger of unleashing similar dangerous deluges.