Three cities in Yemen have now run out of clean water because a blockade by the Saudi-led coalition has cut all imports of fuel needed for pumping and sanitation, the Red Cross says.
The cities of Taiz, Saada and Hodeidah are now deprived of clean water and sanitation, according to the International Committee of the Red Cross (ICRC), putting close to 1 million people at risk of a renewed cholera outbreak.
Yemen was already emerging from the world's worst cholera outbreak in modern times.
"We're very scared that cholera might come back," said Iolanda Jacquemet, an ICRC spokeswoman in Geneva.
"If these water treatment plants and sewage plants stop working, it can only bring cholera back and other water-borne diseases."
Other cities, including the capital Sanaa, are expected to run out in two weeks if the Saudi coalition does not lift its blockade of Yemen.
A US- and UK-backed Saudi coalition has closed much of Yemen's air, land and sea access for the past 12 days, following the interception of a missile fired by Houthi rebels in Yemen towards the Saudi capital.
The Saudis say they are trying to stem the flow of arms from Iran to the Houthis.
Meanwhile, Amnesty International says the US, UK and France risk being complicit in war crimes if they do not cease supplying arms and military support the Saudi coalition as it continues its deadly blockade of Yemen.
"The looming prospect of famine is becoming a reality because of the new restrictions by the Saudi Arabia-led coalition, which appear to amount to collective punishment of Yemen's civilians," said Philip Luther, research and advocacy director for the Middle East and North Africa at Amnesty International.
Amnesty is demanding all countries immediately halt the flow of arms and military assistance to the Saudi coalition for use in Yemen, including any equipment or logistical support being used to maintain the blockade.
The United Nations says 7 million people in Yemen are already in "famine-like conditions", and the UN has said that number could rise to over 10 million if Yemen does not get food and nutritional supplies fast.
Famine is only officially declared after an inspection team has carried out a formal survey on the ground, so there is no guarantee that famine is not already underway.
"There may be, as we speak right now, famine happening," UN humanitarian spokesman Jens Laerke told a UN briefing in Geneva.
"And we hear children are dying. There is excess mortality as a consequence of under-nourishment."