"Elderly and handicapped people who are isolated and who have problems tend not to go out of the house, and so get fewer visitors due to the cold," said Francoise Weber of the French national health surveillance institute.

It reported "a significant increase" in elderly and other vulnerable people being admitted to hospital, as temperatures drop to as low as minus 20 degrees Celsius (minus four Fahrenheit) in some parts of France.

In a radio message broadcast since Friday, the health ministry appealed to citizens to show "vigilance and solidarity" and ensure that elderly or chronically ill neighbours are okay.

Besides hypothermia and frostbite, cold winter weather can weaken a person's cardiovascular and respiratory systems, leaving the lungs more sensitive to infection, the health institute said.