Peru has declared a state of emergency after hundreds of children died from freezing conditions that have seen temperatures across much of the South American country plummet to a 50-year low.

In 16 of Peru's 25 regions, temperatures have fallen below -24C.

Four hundred and nine people, most of them children, have already died from the cold, with temperatures predicted to fall further in coming weeks.

Worst hit are Peru's poorest and most isolated communities, which are already living on the edge of survival in remote Andean mountain villages more than 914m above sea level. Although those living at such high altitude would expect temperatures to drop below zero at this time of year, officials say many are unable to withstand the extreme cold they are now experiencing.

"Over the past three or four years we have seen temperatures during the winter months get lower, and people are unable to survive this," said Silvia Noble, from Plan Peru.

"This cold weather is now extending into areas that never saw these low temperatures before and children and elderly people are especially at risk as they are not physically strong enough to last month after month of sub-zero conditions."

Last December, reporters visited high-altitude farming communities in Huancavelica - one of the areas worst hit by the current cold snap - to find families already struggling with rising child mortality fuelled by malnutrition, poverty and what they say are increasingly erratic and unreliable weather patterns.

Officials say these mountain villages are now racked with pneumonia, chronic respiratory illnesses and hunger.

The freeze is also killing hundreds of alpaca. Farmers are struggling to keep livestock alive because of frozen water points and a lack of food, which could have severe repercussions on the ability of families to see out the winter.

The declaration means authorities can get emergency funds to provide medicine, blankets and shelter to those most at risk.