The number of infants and children who die before reaching the age of five has dropped from 12 million in 1990 to 6.9 million in 2011, according to the UN Children's Fund, UNICEF. The number of lives saved has sped up over the past decade.
However, "any satisfaction at these gains is tempered by the unfinished business that remains," said UNICEF's executive director, Anthony Lake. Some 19,000 children are still dying each day from largely preventable diseases.
The geographic and social disparity also worries UN experts, who called for devoting more resources to at-risk regions and boosting investment in maternal and child health, along with education for girls.
The biggest improvement in child health has been recorded in Latin America and the Caribbean, East Asia and the Pacific, Central and Eastern Europe, the Middle East and North Africa. These regions have cut the child death rate by half since 1990.
Under-five deaths are now increasingly concentrated in sub-Saharan Africa and South Asia, which now account for 80 percent of the world total, said the UNICEF report, titled "Committing to Child Survival: A Promise Renewed."
Sub-Saharan Africa has only cut its death rate by 39 percent, and accounts for nearly half the deaths each year. One child in nine in the poorest African region still fails to live past his or her fifth birthday.
In eastern and southern Africa, deaths have been cut through "massive" investment in fighting AIDS, measles and malaria, said the report.
South Asia accounts for about a third of total under-five deaths each year.
India (24 percent of the total), Nigeria (11 percent), Democratic Republic of Congo (seven percent), Pakistan (five percent) and China (four percent) make up half of the total number of world deaths between them.
UNICEF said that poverty is not the only decisive factor in deaths. Children are more likely to die early if they are born in a rural area or if their mother has not had primary education. Conflict and political instability also hit child health prospects.
Some poor countries have made spectacular progress on child health in recent years, UNICEF said. Laos cut its mortality rate by 72 percent between 1990 and 2011, East Timor by 70 percent, Liberia 68 percent and Bangladesh 67 percent.
"A diverse group of countries including Oman, Estonia, Turkey, Saudi Arabia, Portugal, Peru and Egypt, among others, have been able to sustain high annual rates of reduction in under five mortality," said the report.
"Others such as Rwanda, Cambodia, Zimbabwe and Senegal have succeeded in substantially accelerating their rates of reduction in mortality during the last decade."
Infectious diseases such as pneumonia, diarrhea and malaria account for two thirds of child deaths.
About 40 percent of the children who die are struck down in the first month of life. Factors weighing against a child's survival include whether the mother is very young, or has had a poor education, and whether there is poor hygiene or no access to water.
Mickey Chopra, head of UNICEF's health department, said that poorer countries are getting faster access to vaccines developed in the rich world.
Vaccines now reach poor countries in two to five years, against 20 years in the past, he stressed. Governments and health agencies are also negotiating lower prices with pharmaceutical firms.