An estimated five million children under the age of five died in 2021, the United Nations said Tuesday, urging health care investment in underserved communities around the globe to avoid this "immense, intolerable and mostly preventable loss of life."

"Every day, far too many parents are facing the trauma of losing their children, sometimes even before their first breath," said Vidhya Ganesh, a specialist with the UN's children's agency, UNICEF.

"Such widespread, preventable tragedy should never be accepted as inevitable. Progress is possible with stronger political will and targeted investment in equitable access to primary health care for every woman and child," Ganesh added.

Some 2.3 million deaths in 2021 occurred in the children's first month, mostly due to being premature or from complications related to childbirth. After a child's first month, infectious diseases, such as pneumonia, diarrhea and malaria become the biggest threats.

Most deaths, however, can be prevented with improved health care, vaccination, nutrition as well as water and sanitation programs, the report said.

With vaccination campaigns disrupted by the Covid-19 pandemic, immunization rates among children dropped by two million in 2021 compared to 2020, and by six million compared to 2019, an ominous sign for future child mortality.

But there were also signs for optimism.

The global under-five mortality rate fell 50 percent since 2000, while mortality rates in older children and youth dropped 36 per cent, and the stillbirth rate decreased by 35 per cent.

The report highlighted immense inequalities across the world.

Children in sub-Saharan Africa face the biggest risk of dying before their fifth birthday, with 56 percent of the under-five deaths in 2021 occurring in that region.

"Behind these numbers are millions of children and families who are denied their basic rights to health," said Juan Pablo Uribe, Global Director for Health, Nutrition and Population with the World Bank.

"We need political will and leadership for sustained financing for primary health care which is one of the best investments countries and development partners can make."