"Raging conflict and beleaguered health services are threatening the health of tens of millions of people" in the four countries, WHO said in a statement.
The UN health agency asked international donors to cough up the funds needed to provide services such as surgery for those wounded in the conflict zones.
The WHO said the funds would also help make available childhood vaccinations and treatments for non-communicable diseases like cancers, diabetes, heart and lung diseases to communities often forced to do without.
Of the USD 1.0 billion requested, WHO itself requires USD 322.7 million to deliver health services this year to more than 21 million people in the four countries -- most of them women and children, it said.
The rest would help finance the health divisions of other UN agencies and humanitarian organisations, WHO said.
More than half of the requested funds -- USD 687 million -- is needed for Syria, which has been devastated by nearly four years of civil war that has killed more than 210,000 people and driven half the population from their homes.
The number of doctors and other health workers in the country has meanwhile dropped 45 per cent since the conflict began in March 2011, and vaccination coverage has plunged from 90 per cent to only 52 per cent today.
