An intense heatwave across south Asia has killed dozens of people with sustained temperatures in excess of 40C (104F) coinciding with power cuts and Ramadan, when many Muslims avoid eating or drinking water.

At least 65 people have died in Karachi in recent days according to the charitable organisation that runs the central morgue in the Pakistani port city, as volunteers handed out water to labourers and others working outside in temperatures as high as 44C.

Local media reports claimed the death toll could have exceeded 100 in the sprawling megacity of 15 million, where high temperatures are exacerbated by an absence of green space, estimated to make up just 7% of the urban area.

Authorities in Karachi have not confirmed the death toll but urged people to stay indoors and keep drinking water.

Parts of the city have also been suffering from power cuts, particularly early in the morning when more people than usual have been waking to eat before sunrise in line with Ramadan fasting rituals.

During the annual month-long rite, Muslims refrain from eating or drinking anything including water between sunrise and sunset, though children, older people, the sick and pregnant women usually do not participate.

The Edhi Foundation, which runs Karachi’s central morgue, said most of the dead in the city were working-class people from poorer neighbourhoods, including children and elderly people.

“They work around heaters and boilers in textile factories and there is eight to nine hours of [scheduled power outages] in these areas,” Faisal Edhi, the head of the foundation, told Reuters.

Fazlullah Pechuho, the health minister of Sindh province, denied that anyone had died, however, telling the Dawn newspaper: “I categorically reject that people have died due to heatstroke in Karachi.”

A heatwave in 2015 left morgues and hospitals overwhelmed and killed at least 1,200 mostly elderly, sick, and homeless people.

In 2015, the Edhi morgue ran out of freezer space after about 650 bodies were brought in the space of a few days. Ambulances left decaying corpses outside in the sweltering heat.

Extreme temperatures were also recorded in neighbouring India, with parts of Maharashtra state reaching 47C on Monday.

More than 22,000 people died in heatwaves in India between 1992 and 2005, according to government data released last year.

Authorities say that figure has fallen dramatically since as a result of public health campaigns, including training medical staff to diagnose heat-related illnesses and sending colour-coded temperature warnings through the media and WhatsApp.

Last year, about 222 Indians died because of the heat, the government says, down from 1,111 in 2016 and 2,040 the year before.

Temperatures in Karachi are expected to stay above 40C until 24 May, according to weather forecasters. High temperatures were also predicted to last another 48 hours in India.

Reuters contributed to this report