The extreme heat in the northern hemisphere is putting an increasing strain on healthcare systems, hitting those least able to cope the hardest, the World Health Organization said Wednesday.

The WHO said the heat often worsens pre-existing conditions, saying it was particularly concerned about those with cardiovascular diseases, diabetes and asthma.

Millions of people across three continents are enduring a sustained spell of dangerous heat on Wednesday as temperature records tumble.

"Extreme heat takes the greatest toll on those least able to manage its consequences, such as older people, infants and children, and the poor and homeless," said WHO chief Tedros Adhanom Ghebreyesus.

"It also puts increased pressure on health systems," he told a news conference.

"Exposure to excessive heat has wide-ranging impacts for health, often amplifying pre-existing conditions and resulting in premature death and disability."

The WHO was working with the World Meteorological Organization (WMO), its fellow Geneva-based UN agency, to support countries in developing hot-weather action plans to coordinate preparedness and reduce the impacts of excessive heat on health, he added.

Maria Neira, the WHO's public health and environment chief, said the agency was particularly concerned about pregnant women and people with diabetes and cardiovascular diseases, and asthma, as air pollution would be part of the problem.

Local and national governments needed to identify all those potentially at risk, while hospitals should ensure they had an action plan in place, she added.

Neira also said communities needed to get the message out on avoiding sport during the hottest part of the day, finding a cool place indoors, looking out for the vulnerable, and being aware of heat stroke or heat exhaustion.

Experts have blamed the heatwaves on climate change, driven by the burning of fossil fuels releasing the greenhouse gas carbon dioxide into the atmosphere.

"That will be helping us to reduce the heatwaves in a very important way."

City officials needed to think through their urban planning to ensure people had refuges in times of extreme heat, she added.

The UN's WMO weather agency has said repeated high overnight temperatures are a particular health risk because the body is unable to recover from hot days, leading to more heart attacks and deaths.

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