Deadly heatwaves are scorching cities on four continents as the Northern Hemisphere marks the first day of summer, a sign that climate change may again help to fuel record-breaking heat that could surpass last summer as the warmest in 2,000 years.

Record temperatures in recent days are suspected to have caused hundreds, if not thousands, of deaths across Asia and Europe.

In Saudi Arabia, nearly two million Muslim pilgrims are finishing the haj at the Grand Mosque in Mecca this week. But hundreds have died during the journey amid temperatures above 51 degrees Celsius, according to reports from foreign authorities.

Countries around the Mediterranean have also endured another week of blistering high temperatures that have contributed to forest fires from Portugal to Greece and along the northern coast of Africa in Algeria, according to the US National Oceanic and Atmospheric Administration's Earth Observatory.

In Serbia, meteorologists forecast temperatures of around 40C this week as winds from North Africa propelled a hot front across the Balkans. Health authorities declared a red weather alert and advised people not to venture outdoors.

Belgrade's emergency service said its doctors intervened 109 times overnight to treat people with heart and chronic health conditions.

In neighbouring Montenegro, where health authorities also warned people to stay in the shade until late afternoon, tens of thousands of tourists sought refreshment on the beaches along its Adriatic coast.

Parts of the US Northeast and Midwest are also wilting under a heat dome, with more than 86 million people under a heat alert on Thursday, according to the National Weather Service.

A heat dome occurs when a strong, high-pressure system traps hot air over a region, preventing cool air from getting in and causing ground temperatures to remain high.

India's summer period lasts from March to May, when monsoons begin slowly sweeping across the country and breaking the heat. But New Delhi on Wednesday registered its warmest night in at least 55 years, with India's Safdarjung Observatory reporting a temperature of 35.2 C at 1am. (Reuters)