Entitled "Meeting the Millennium Development Goals," their report is a mid-course appraisal of a 25-year plan ending in 2015 to dramatically improve sanitation conditions and access to clean drinking water throughout the world.
The report covers the period from 1990-2002.
The World Health Organisation (WHO) and the United Nations Children's Fund (UNICEF) say the plan is "on track" for drinking water, but that half a billion people - mainly in rural Africa and Asia - fell short of the sanitation targets set for 2002.
The world's poorest and least developed nations, notably those in sub-Saharan Africa, suffer most from the diseases caused by poor hygiene, which hit children hardest of all.
"The growing disparity between the have and the have-nots in terms of access to basic services is killing around 4000 children every day and underlies many more of the 10 million child deaths every year," Bellamy said. "We have to act now to close this gap or the death toll will certainly rise," she added.
Diarrhea kills 1.8 million persons every year, mainly children under five, and the search for scarce drinking water results in 40 billion hours of lost work in Africa alone, the report says.
But there are also "worrying trends," aggravated by rapid industrialisation in many industrialised countries, where the percentage of people with access to clean drinking water and basic sanitation actually dropped by two percent between 1990 and 2002.
In the ex-Soviet Union, for example, only 83 percent of the population live in conditions of adequate sanitation.
"Water and sanitation are among the most important determinants of public health," noted WHO Director-General Lee Jong-wook. "Wherever people achieve reliable access to safe drinking water and adequate sanitation they have won a major battle against a wide range of diseases," he added.
The report did underline some encouraging signs. Efforts to reduce the number of people without access to clean drinking water to 800 million by 2015 are on track, it said, noting that an additional 1.1 billion people have gained such access in the last 12 years.
Progress on this front has been especially rapid in Asia, where the percentage of the population with access to potable water has increased from 77 to 83 since 1992. But there are still 675 million people in Asia, the report estimates, that draw their drinking water from unsafe sources such as polluted rivers and lakes.