Syrians living in areas affected by the nearly two-year conflict have seen their water supplies cut by one third, putting children at especially high risk of disease, the United Nations said on Friday.

The results of the first UN Fund for Children nationwide assessment of water and sanitation since hostilities began revealed that populations in contested areas have only 25 litres (5.5 gallons) of water a day, compared with 75 litres two years ago.

Of the estimated four million people in need, 50 percent are children, UNICEF said.

In regions where fighting has been the fiercest, including in Deir Ezzor province in the east, water was being pumped at just 10 percent of pre-crisis capacity.

The other hardest-hit areas are rural areas of Damascus province, the provinces of Idlib, Aleppo and Raqa along the northern border with Turkey and the central province of Homs.

"National production of water treatment chemicals almost ceased because of conflict, increasing the risk that tap water is contaminated," said UNICEF's Syria representative Youssouf Abdel-Jelil.

The agency emphasised that children were particularly vulnerable to waterborne disease because of protracted water cuts, damage to sanitation systems and a lack of access to basic hygiene.

UNICEF reported that conditions were especially dire for displaced people living in collective shelters, including in 1,500 schools where they have taken refuge.

"Living conditions are often unsanitary due to the lack of toilets, showers, hygiene items such as soap, and rationed access to water -- often less than 10 litres per person per day," it said.

The agency noted that many families are forced to buy water from mobile tankers, the quality of which is either poor or questionable. Moreover, the $30 per month cost is far above the means of most families.

UNICEF is struggling to meet its goal of providing 750,000 people in Syria with safe drinking water, soap, hygiene kits, toilets and bathrooms by June, because of a funding gap of 80 percent.