The WHO's annual World Health Statistics recorded about nine million deaths of under five year-olds in 2007, 28 percent less than the 12.5 million who died in 1990 but still far short of the Millennium Development Goals.
Child mortality is one of the MDGs adopted by UN member states, with the aim of cutting infant deaths by two-thirds between 1990 and 2015.
"The decline in the death toll of children under five illustrates what can be achieved by strengthening health systems," said WHO health statistics director Ties Boerma.
He highlighted the impact of insecticide treated mosquito nets now distributed in many poor countries to combat malaria, rehydration salts for diahorrea and increased access to vaccines in improving the health of under fives.
But that progress was still slow and sparse, especially in poor countries and in Africa.
In low income countries, 22 dollars per capita goes to health care, compared to 4,012 dollars in wealthy nations, according to the WHO.
While there are 32 doctors for 10,000 European Union inhabitants, in poor countries the ratio drops to an average of two doctors per 10,000 people.
Pneumonia and diarrhoea kill 3.8 million infants every year, even though both conditions are treatable, the WHO said.
Another development goal, maternity mortality, remains largely unchanged since 1990, with a global average death rate of 400 maternal deaths per 100,000 births a year, and more than double that rate in sub-Saharan Africa.
"There needs to be more effort to strengthen health systems in countries affected by high levels of HIV/AIDS, economnic hardship or conflict," said Boerma.