The world should prepare to face increasingly intense heatwaves, the United Nations warned on Tuesday, as countries across the Northern Hemisphere reeled from soaring temperatures.

"These events will continue to grow in intensity, and the world needs to prepare for more intense heatwaves," John Nairn, a senior extreme heat advisor at the UN's World Meteorological Organization (WMO), told reporters in Geneva.

His comments came as Europe was bracing for new high temperatures on Tuesday under relentless heatwaves and wildfires that have scorched swathes of the Northern Hemisphere, forcing the evacuation of 1,200 children close to a Greek seaside resort.

Health authorities have sounded alarms from North America to Europe and Asia, urging people to stay hydrated and shelter from the burning sun, in a stark reminder of the effects of global warming.

Heatwaves are amongst the deadliest natural hazards, with hundreds of thousands of people dying from preventable heat-related causes each year, Nairn said.

Heat is a rapidly-growing health risk, due to burgeoning urbanisation, an increase in high temperature extremes, and demographic changes in countries with ageing populations.

Nairn said that the number of drawn-out and simultaneous heatwaves in the Northern Hemisphere