The International Committee of the Red Cross (ICRC) said the cholera outbreak "continues to spiral out of control" since it erupted in April.
"Today, over 300,000 people are suspected to be ill," it said in a Twitter post.
More than 7,000 new cholera cases are being recorded each day in the capital Sanaa and three other areas.
The highly contagious disease is treatable, but the collapse of Yemen's infrastructure following more than two years of a Saudi-led war against Houthi rebels in the country has created a "perfect storm for cholera", the World Health Organization has said.
The war has left less than half of the country's medical facilities functional, with aid groups diverting funding from fighting malnutrition to battle the disease – raising the risk of famine.
Much of the $1.1 billion in aid pledged by donor governments in April to deal with the population's needs has yet to be distributed, leaving relief agencies struggling to obtain funds, the UN's humanitarian coordinator in Yemen, Jamie McGoldrick, said.
"Humanitarian organisations have had to re-programme 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."
Two-thirds of Yemen's population, around 17 million people, are unsure of where their next meal will come from, according to the World Food Programme.
The war in Yemen has left more than 10,000 dead since it escalated with the Saudi-led coalition intervention in March 2015.
On Monday, Britain's High Court ruled that UK arms sales to Saudi Arabia were lawful, despite concerns over the civilian death toll during Riyadh's bombing campaign in Yemen.
The UK government has licensed around £4.1 billion ($5.3bn) of weapons to the Middle East since the election in May 2015.
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