NEW DELHI — An early-season heatwave pushed daytime temperatures above 46 degrees Celsius across a 1,400-mile band of South Asia on Thursday, killing at least 71 people across India, Pakistan and Bangladesh, prompting school closures from Sindh to West Bengal and overwhelming hospital wards in cities where the pre-monsoon peak normally arrives weeks later.

The India Meteorological Department issued a “red” extreme-heat warning for 14 districts across Rajasthan, Uttar Pradesh, Bihar and West Bengal, the agency’s highest alert level, and the Pakistan Meteorological Department issued matching warnings for upper Sindh, southern Punjab and parts of Balochistan. Bangladesh’s meteorological office said Dhaka’s high of 41.8 degrees Celsius on Thursday afternoon — about 107 degrees Fahrenheit — was the city’s hottest April temperature on record and that conditions were expected to worsen through the weekend before a possible westerly trough offered partial relief Monday.

The early arrival of dangerous heat in mid-April, six to eight weeks before the southwest monsoon typically breaks the dry season, has placed enormous strain on health systems already stretched by routine pre-monsoon respiratory and diarrheal caseloads. Officials in three Indian states reported that emergency rooms were running at 130 to 150 percent of designed capacity, and the Karachi city government said its public-hospital trauma units had recorded a 60 percent week-on-week increase in heat-related admissions.

“What we are looking at this week is not a single bad day. It is the front end of a heat dome that has parked itself over the subcontinent and that is not going to move until something else moves it,” said Dr. Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology in Pune, in an interview. “April heat at this intensity is no longer an outlier. It is the new face of the pre-monsoon.”

In Jacobabad, in upper Sindh, a city that has become a familiar reference point in global discussions of the upper limits of human heat tolerance, the Pakistan Meteorological Department recorded a high of 47.6 degrees Celsius on Wednesday and a low of 32.1 degrees overnight — meaning the wet-bulb conditions in the city did not drop into safely survivable territory for unacclimatized outdoor workers for any sustained period in 24 hours. Local hospitals reported 11 deaths Wednesday and Thursday attributed to heat stroke, including two laborers at a brick kiln on the city’s outskirts and a 78-year-old woman whose neighborhood had been without grid power for 14 hours during the day Wednesday.

“We are telling people to stay indoors, but indoors is not always cooler,” said Saira Tabassum, a senior physician at Jacobabad’s civil hospital who spoke by phone Thursday evening. “If the power is off, if the roof is tin, if there are 11 people in two rooms, indoors is the dangerous place. There is nowhere in this city to send people right now.”

India’s home ministry said the Central Water Commission had moved to release additional water from reservoirs in Madhya Pradesh and Uttar Pradesh to maintain drinking-water supply in cities where municipal storage had fallen below seasonal norms, and that the National Disaster Management Authority had pre-positioned ice, oral rehydration salts and portable cooling equipment in 41 districts. Heat-action plans drawn up after the 2010 and 2015 heatwaves were activated in full for the first time this season in nine state capitals.

In West Bengal, the state government ordered all schools through grade 12 closed through Tuesday after a 9-year-old boy collapsed during a morning assembly at a school in Bardhaman district Wednesday and was pronounced dead on arrival at hospital. Chief Minister Mamata Banerjee said in a statement that the state would extend the closure if conditions warranted. Neighboring Odisha and Jharkhand followed with similar orders Thursday afternoon, and the National Capital Region’s school administrations said they were monitoring conditions hour by hour ahead of decisions expected Friday morning.

Pakistan’s federal climate change ministry said Thursday that it was coordinating with provincial governments on a tiered response and that the prime minister’s office had directed the National Disaster Management Authority to keep cooling centers in Karachi, Hyderabad, Multan and Sukkur open around the clock. Federal Climate Change Minister Romina Khurshid Alam, in remarks to reporters in Islamabad, said the country’s grid was straining under air-conditioning load that had ordinarily not peaked until late May.

“We are managing a May-June electricity demand curve with an April-level water table,” Alam said. “That is a combination this system was not built for, and we are being honest with the public about that.”

Bangladesh’s environment ministry warned that the simultaneous combination of high temperatures and low river flow in the Padma and Jamuna systems was raising salinity in coastal aquifers and threatening drinking-water supplies in Khulna and Barishal divisions. The country’s Cyclone Preparedness Programme, normally activated for tropical-storm season, was repurposed Thursday to deploy 18,000 trained community volunteers as heat-response monitors in the most exposed districts.

The heatwave has also rippled through agriculture. Wheat harvesting, normally well underway across Punjab and Haryana in mid-April, has been accelerated and in some districts compressed into pre-dawn and post-sunset windows to spare laborers the worst of the daytime heat. Indian state agriculture officials in Punjab said early reports suggested grain yields could be reduced 4 to 7 percent in heat-affected districts because of premature ripening and reduced grain filling, though final assessments would take several weeks. Pakistani officials said similar concerns extended to mango orchards in southern Punjab, where heat stress during the flowering and fruit-setting stage in March had already raised concerns about the export crop.

Public-health officials across the region said the most dangerous period was likely still ahead. The South Asian heat season typically peaks in late May or early June before monsoon onset, and the current event, while extraordinary for April, would not break the underlying seasonal trajectory. Climate scientists at the Indian Institute of Science and at Pakistan’s Centre for Climate Research and Development in Lahore said attribution analyses of the current event were already underway, with preliminary findings expected within two weeks.

“What we are going to find, almost certainly, is that an event of this intensity in April has been made several times more likely by warming,” said Dr. Fahad Saeed, a climate scientist at Climate Analytics who has co-authored multiple attribution studies on South Asian extreme-heat events. “What we are also going to find is that the difference between a survivable day and an unsurvivable day in this region is a few hours of grid power and a roof that does not turn into an oven. That is where the policy fight is.”

The World Health Organization’s South-East Asia regional office in New Delhi said it was working with member-state health ministries to compile real-time mortality and morbidity data and that an interim regional advisory on heat-stroke triage and emergency-room surge protocols would be issued by the weekend. Officials said additional measures, including possible cross-border coordination on power and water sharing in the most-stressed districts, would be announced in the coming days.