The International Committee of the Red Cross is buying 750,000 litres of diesel fuel to help provide clean water for a million people living in Yemen.
Nine cities ran out of clean water this month because a blockade by a Saudi-led coalition prevented imports of fuel needed for pumping and sanitation.
The ICRC's fuel purchase will allow the water systems in two cities, Hudaydah and Taiz, to operate for a month.
Remaining fuel stocks, some available only on the black market, have rocketed in price leaving local water boards unable to afford them.
The coalition, which has backed the Yemeni government in its war with the rebel Houthi movement since 2015, closed Yemen's borders to stop alleged weapons smuggling on 6 November after a missile was fired at the Saudi capital Riyadh.
Yemen is reliant on imports for more than 90% of its needs, and it faced the largest food security emergency in the world even before the blockade was tightened.
More than 2.5 million people across Yemen are currently without access to clean water as a result, according to the ICRC.
Without access to clean water and sanitation, Yemenis living in crowded urban areas are at risk of a renewed cholera outbreak and other water-borne diseases.
In Hudaydah, untreated sewage has been flooding the streets regularly.
Some 17 million people in Yemen do not know where their next meal is coming from and seven million are totally dependent on food assistance.
Severe acute malnutrition is threatening the lives of almost 400,000 children.