The levels of malnutrition among children fleeing Somalia's drought could lead to a "human tragedy of unimaginable proportions", the UN refugee head Antonio Guterres has said. Young children are dying on their way to or within a day of arrival at camps in Ethiopia and Kenya, the UNHCR says. It estimates that a quarter of Somalis are either displaced within the country or living outside as refugees. The worst drought in 60 years has been compounded by the violence in Somalia. "It's so extreme," said UNHCR spokeswoman Melissa Fleming. "Our people are saying they've never seen anything like it." The warning comes as the UK aid agencies Oxfam, Save the Children, and the Red Cross launch emergency appeals in response to the food crisis which is affecting more than 12 million people in the Horn of Africa. The agencies are collectively asking for nearly $150m (£93m). The UNHCR says the need for food, shelter, health services and other life saving aid is urgent and massive. Life-long impact The agency says more than 50% of Somali children arriving in Ethiopia are seriously malnourished. In Kenya, that figure is between 30% and 40%. "What is the most tragic for us to witness, is that there are children who do arrive in such a weakened state that despite our emergency care and therapeutic feeding, they're dying within 24 hours," Ms Fleming told a press briefing in Geneva. "We estimate that one quarter of Somalia's 7.5 million people are now either internally displaced or are living outside the country as refugees," she said. The UNHCR recently opened a third camp in south-eastern Ethiopia, which is quickly reaching its capacity of 20,000, and is now planning further camps. A relief plane chartered by the agency is flying to Addis Ababa on Tuesday and a convoy of 20 trucks carrying tents and other aid is on its way as well. In north-east Kenya's Dadaab refugee camp, some 1,400 refugees are arriving every day. Aid agencies fear numbers could rise to half a million. Badu Katelo, Kenya's Commissioner for Refugee Affairs, said food and water distribution, shelter and space were all over stretched and that the security situation was getting worse. "We would like to see a vibrant, committed intervention from the international community," he said. The BBC's Ben Brown, at the Dadaab camp, says infant mortality has risen threefold, with many children under the age of five dying within a few days of arrival. Families have walked for days with hardly any food or water to reach the camp, says our correspondent - some say they were robbed or raped on the way or attacked by animals.