The world only has a "window of three to four months" to save millions in the two countries, according to the humanitarian organisation.
There is a window of only three to four months to avert mass starvation in Yemen and Somalia, as war and drought wreck crops and block deliveries of food and medical care, the International Committee of the Red Cross (ICRC) said on Wednesday.
The aid agency still needs $300 million to bring emergency assistance to a total of 5 million people in Yemen, Somalia and northeast Nigeria as well as areas of South Sudan, where famine has already been declared.
"We have probably a window of three to four months to avoid a worst case scenario," Dominik Stillhart, the ICRC's director of operations worldwide said.
Millions facing famine
More than 20 million people are facing famine in Yemen, Somalia, South Sudan and northeast Nigeria, say aid agencies.
Cholera, which can be deadly for children, is on the rise in Somalia, where drought is killing livestock and driving people to flee in search of water, said Bruce Orina, ICRC deputy regional director for Africa.
At least 300,000 malnourished children are trapped by fighting in northeast Nigeria.
"If we do not avert these water-borne diseases then we are exposing these huge numbers of children to a likely scenario of death," Orina said.
The ICRC has received $100 million towards the $400 million needed for its operations in the four countries this year.
The United Nations has appealed for about $5.6 billion, bringing total funding needs to $6 billion, ICRC's Stillhart said.
"There are significant needs and, of course, there are serious concerns in terms of having funding available sufficiently fast in order to avert what I said is large-scale starvation," he said.
The United States, whose average contribution finances about one-quarter of ICRC field operations, has yet to donate, he said, noting a top ICRC official was to speak about the famine to the US Senate Foreign Relations Committee on Wednesday.
"In 2011 the response was too slow and too late, leading to the starvation of 260,000 people in Somalia alone," he warned.
Robert Mardini, ICRC regional director for the Middle East, said that an average of 20 people die each day from curable disease or war wounds in Yemen where less than half of health facilities still function.
An ICRC team on Tuesday reached Taiz, a city of 400,000 "on the verge of collapse," Mardini said. "Food is extremely scarce in the city and it's not affordable for the vast majority of people."