The Yemeni cities of Taiz, Saada and Hodeidah have run out of clean water due to a blockade by the Saudi-led coalition, according to the Red Cross.
There are fears of a renewed cholera outbreak, following on from a recent outbreak that was the worst seen in modern times.
Aid groups are describing the blockade as a humanitarian crisis, and have had much of their access to civilian populations cut off.
A number of other cities, including the capital Sanaa, are also running low on water, and are expected to run out within two weeks if the blockade is not lifted.
The blockade comes in response to a missile that Houthi rebels in Yemen fired at the Saudi capital.
In response, a Saudi-led coalition, with the backing of the US and the UK, has shut down air, land and sea access to Yemen for the past 12 days.
Saudi Arabia says the blockade is intended to stop the flow of arms from Iran, its regional rival, to the Houthis.
About 20.7 million people are in need of aid, according to the UNHCR, with as many as 2 million having been displaced.
The blockade is the latest part of a proxy war between Iran, backing the Houthi rebels, and Saudi Arabia, backing the exiled government of Yemen.