While some nations -- notably China -- have made significant progress, far more, especially in sub-Saharan Africa, have either stagnated or lost ground, said the report.
The series of studies, published by the British medical journal The Lancet, is the second so-called "Countdown" assessment on how well the world's nations are doing in meeting eight Millennium Development Goals (MDG) adopted in 2000.
One of the objectives is to slash the death rate among children under five by two thirds before 2015, using the beginning of the century as a benchmark. Another seeks to improve maternal health.
The report, which focuses on the 68 countries accounting for 97 percent of maternal and child deaths worldwide, makes for grim reading.
Only 16 of these high-risk nations are on track to meet their goals. Twenty-six have made no progress whatsoever, and 12 -- including Cameroon, Botswana, Kenya and South Africa -- have actually slid backwards.
The bottom line is that more than 10 million youngsters die before the age of five every year, most from preventable causes, the study says.
In at least 15 African countries, one out of four or five children never sees his or her fifth birthday, according to UN statistics.
In the world's richest nations, the corresponding figure is one out of 200 or less.
"National and global attention to maternal, newborn, and child health is still strikingly inadequate," Richard Horton, editor-in-chief of The Lancet, wrote in a sharply-worded commentary.
"Children and mothers are dying because those who have the power to prevent their deaths choose not to act," he said.
Seven of the 16 nations that are succeeding in the fight against child mortality were already on track when the countdown was launched in 2005, the study showed.
They include Brazil, Bangladesh, Mexico and Indonesia, some of the world's most populous nations.
Six others were included in the Countdown process -- initiated by UN and public-health scientists to spur progress -- for the first time this year, including Eritrea, Peru, and Morocco.
And three made enough progress to move from a negative assessment in 2005 to a positive one in 2008, most notably China.
A reduction of child mortality by 25 percent in five years in Tanzania showed limited resources was not a barrier to improvement.
"Despite the challenges of endemic poverty, less than optimum infrastructure, an insufficient health workforce, high fertility rate, and severe HIV burden" -- the factors contributing to unacceptably high death rates among children throughout the continent -- "substantial progress was made," the researchers say.
Besides health systems, Horton highlighted two other "vital issues": nutrition and the collection of health data.
Maternal and child undernutrition account for 20 percent of maternal deaths and 35 percent of under-five deaths, he said. "But at the same time nutrition only accounts for eight-to-13 percent of total aid flow."
The researchers also found that routinely scheduled immunisation and pre-natal care proved more efficient than relying on 24-hour clinical services for emergency care at birth, or care of ill newborn babies and children.
Another critical focus is working to prevent mother-to-child transmission of HIV, they conclude.
The 2008 report warns that the next two years are critical if the Millennium health goals are to be met by 2015, and called especially for targeted funding and a strengthening of health systems.
UN chief Ban Ki-moon will co-host a high-level meeting in September on the lagging implementation of the poverty-reduction goals, particularly in Africa.
The eight MDGs are: eradicating extreme poverty and hunger, achieving universal primary education, promoting gender equality, reducing child mortality, improving maternal health, combating diseases such as HIV/AIDS, ensuring environmental sustainability and creating global partnerships for development.