"We are no longer talking about a humanitarian crisis or a humanitarian emergency," said Jens Oppermann, the country director of Action Against Hunger (Action Contre La Faim, ACF).
"We are seeing this as a humanitarian catastrophe," he told AFP.
Thousands of Somalis have fled in recent months to neighbouring Ethiopia and Kenya in search of food and water, with many dying along the way, as the region suffers what the UN has described as the worst drought in decades.
Many too have risked conflict by fleeing into Somalia's capital Mogadishu in a desperate search for aid.
Food prices in Somalia have soared by 270 percent in a year, he said.
"We see people coming into Mogadishu in a state that we have not seen in this form before," Oppermann added, describing "unimaginable suffering beyond the scale of what is acceptable."
"We are not able to provide enough assistance to everyone," he said.
ACF has been working in Somalia since 1992.
Meanwhile Doctors without Borders (MSF) said they were boosting efforts to support refugees fleeing across the border into Kenya after assessments found "alarmingly high rates of malnutrition."
"I expected to find a difficult situation but not a catastrophic one," said Anita Sackl, who coordinates nutritional assessments for MSF.
"The majority of new arrivals actually fled because they had nothing to eat, not just because their country has been at war for decades," she added.
Refugees are waiting for 40 days before being registered, receiving only two days of rations for that period, MSF said in a statement.
In Kenya's Dadaab refugee camp, 37.7 percent of children under five checked by MSF teams were suffering from acute malnutrition, with 17.5 per cent of those facing a "high risk of death."