"Ongoing air strikes, ground fighting and fuel shortages mean that an additional three million Yemenis are now without drinking water, raising the total number of Yemenis without a clean water supply and sanitation to at least 16 million," the Britain-based organisation said.
"This is equivalent to the populations of Berlin, London, Paris and Rome combined," Oxfam's Yemen director Grace Ommer said in a statement.
Even before the escalation in fighting, half the population of the impoverished nation had no access to clean drinking water, the group said.
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A coalition led by Riyadh launched air strikes on March 26 against Shiite Huthi rebels and allied forces loyal to former president Ali Abdullah Saleh, in a bid to restore UN-backed President Abedrabbo Mansour Hadi.
The rebels overran the capital in September and swept southwards, forcing Hadi to flee to Saudi Arabia and sparking fierce clashes between his supporters and opponents.
Weeks of air strikes and ground fighting have damaged and disrupted large parts of the water network, Oxfam said.
"People are being forced to drink unsafe water as a result of the disintegration of local water systems, bringing the real risk of life-threatening illnesses, such as malaria, cholera, and diarrhoea," the organisation said.