Some 2.3 million Yemenis have been uprooted from their homes many fleeing violence, others flocking to areas where water is available.
The United Nations says the humanitarian situation in Yemen, the poorest country on the Arabian Peninsula, its plight made worse by a Saudi blockade of its ports, is "critical".
Even before the latest crisis began, only about half the population had access to drinking water and sanitation, such as pit latrines, in one of the world`s driest countries.
Fighting has damaged electricity lines and power plants required to pump water to communities. Fuel for backup generators is scarce because of the Saudi blockade.
As a result, 80 percent of the population is now grappling with a lack of water and poor personal hygiene, the head of UNICEF`s water and sanitation operations in the country said.
"About 20.5 million people are struggling to get drinking water out of a total population of between 25 and 26 million," Mahboob Bajwa told the Thomson Reuters Foundation by telephone from Sanaa.
"People are going to areas where there is some water. They are trying to buy fuel on the black market to run their water supply systems," he said in an interview late on Tuesday.
Bajwa, based in Yemen for three years, said that in some areas water is available for about 30 minutes a day, compared with 2 hours before the Saudi-led campaign began.
"Before, people were getting 20 litres per capita per day. Now it is 1 or 2 litres per capita per day. There`s a huge compromise on their personal hygiene because of a lack of water available to them," he said.
For the past three months, UNICEF has been providing local water authorities in 11 cities including Sanaa with fuel to pump water. It has also distributed more than 300,000 hygiene kits of soap, toothpaste and other items to displaced people.
"Poor hygiene, poor water quality, open defecation have contributed to high levels of chronic malnutrition in this country. You can imagine the number of cases will definitely increase," Bajwa said.
"We fear that, in particular, children under the age of five are going to feel the huge impact of poor personal hygiene of their mothers and themselves."
The World Health Organization (WHO) estimates that half of all malnutrition is linked to repeated diarrhea or intestinal worm infections due to unsafe water, inadequate sanitation or insufficient hygiene.
In children, nutrients are not absorbed in the body, but diverted to fight such infections, often resulting in stunting.
UNICEF says 537,000 children under age five are at risk of severe acute malnutrition a threefold increase from 160,000 in March.
Close to 1.3 million children under five are moderately malnourished compared with 690,000 before the current crisis.
Bajwa said if fuel was allowed into Yemen, fuel-reliant water systems could be up and running within two weeks.
"The situation would immediately change," he said.
Saudi Arabia has been leading the Arab military intervention to try to restore Hadi`s government, now based in Aden, and fend off what it sees as creeping Iranian influence.
© Thomson Reuters 2015
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