The International Committee of the Red Cross said the cholera epidemic "continues to spiral out of control" since it erupted in April.
"Today, over 3,00,000 people are suspected to be ill. More than 1,600 have died," it said in a Twitter post.
ICRC regional director Robert Mardini said about 7,000 new cholera cases were being recorded daily in the capital Sanaa and three other areas.
The collapse of Yemen's infrastructure after more than two years of war between the Saudi-backed government and Shiite rebels who control Sanaa has made for a "perfect storm for cholera", according to the World Health Organization.
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Although the disease is easily treatable, doing so in Yemen has proved particularly difficult. The war has left less than half of the country's medical facilities functional.
The WHO's own figures for the outbreak list 2,62,649 suspected cases and 1,587 deaths as of July 2, in 21 of 23 Yemeni governorates. It is expected to update those numbers shortly.
The battle against cholera has caused aid groups to pull resources away from fighting malnutrition among Yemen's war- weary people, raising the risk of famine as they struggle to find funds, a UN official warned last week.
"Humanitarian organisations have had to reprogramme their resources away from malnutrition and reuse them to control the cholera outbreak," he said in Sanaa on Thursday.
"And if we don't get these resources replaced, then using those resources for cholera will mean that food insecurity will suffer.
"We're trying to do our best, but it's very much beyond what we can cope with."
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