The UN said Tuesday it had received just 13 percent of the $1.6 billion needed for aid to Yemen, where three months of fighting has forced more than one million people to flee their homes.
"The operations are critically underfunded," Jens Laerke, spokesman for the UN humanitarian agency told reporters in Geneva.
The UN launched its $1.6-billion (1.45-billion euro) appeal to help millions of people in war-ravaged Yemen last month, but has so far received just $209 million of that, he said.
Nearly a third of the funds received had come from the United States, which pitched in $63 million, while Japan had handed over $19 million and the European Commission had given $15 million.
Saudi Arabia, behind the devastating airstrikes pounding Yemen since late March, has pledged $274 million towards the UN appeal, but none of that cash has yet materialised.
"As of this morning, no cash has been transferred yet," Laerke said.
The Saudis have pledged almost as much in humanitarian aid outside the UN appeal system, but it remained unclear if any of that money had materialised.
Laerke insisted that the UN's humanitarian operation in Yemen "does not stand and fall" with the money pledged by the Saudis.
The $1.6 billion requested is meant to cover aid, including food, water and shelter, to 11.7 million of the most vulnerable people in need.
That is meanwhile only just over half of the some 21 million people, or 80 percent of the population, estimated to need assistance.
The nine-nation Saudi-led coalition launched air strikes on Yemen on March 26 to halt an advance by the Iran-backed Huthis who drove President Abedrabbo Mansour Hadi into exile.
More than 3,000 people have been killed in the fighting since then, around half of them civilians, according to UN numbers.
More than one million people have been displaced inside the country since March, joining the more than 300,000 displaced there before the fighting began, and more than 46,000 people have fled the country, according to the UN refugee agency.
While waiting for donors to step up to the plate, UN agencies struggling to address the towering needs in Yemen were drawing from their reserves, but Laerke warned the cash would not last for long.
"Clearly there is a massive humanitarian crisis going on," he said.
Antoine Grand, head of the International Committee for the Red Cross's delegation in Yemen, meanwhile warned in a telephone briefing that the situation was "catastrophic in general" and was "deteriorating by the day".