Large areas of Kenya, Somalia, Ethiopia are affected and the DEC appeal will also include South Sudan, set to become the world's newest country on July 9, the Disasters Emergency Committee (DEC) said.

The group, which represents 14 aid agencies including Oxfam, the British Red Cross and Save the Children, said appeals would be broadcast on Friday by all Britain's terrestrial television channels.

"Slowly but surely, these people have seen their lives fall apart -- crops, livestock and now their homes have been taken by the drought," DEC chief executive Brendan Gormley said.

"We have a duty to help quickly before the situation spirals out of control."

DEC said more than 1,300 people a day, most of them children, are arriving in the Dadaab refugee camp in eastern Kenya after trekking across parched scrubland from Somalia.

The camp is already the world's largest refugee camp with a population of 350,000, DEC said.

"Of course these people need a long-term solution with investment and political will but right now it's about preventing a tragedy," Gormley said.

"Many of these are a forgotten people, caught in the midst of conflict in Somalia and an ever-worsening environmental crisis."

Agencies will be working with local partners to access remote areas across East Africa with food, water, treatments for malnourished children and medicine, the DEC said.

Britain took a lead among the worlds governments on Sunday with a pledge of �38 million ($61 million, 42 million euros) to the UN World Food Programme, which will provide the food aid that many of DEC's members will distribute.

"We need to plan for the long term to help people rebuild their communities when the situation improves," Valerie Amos told reporters in Addis Ababa.

She cited better access to water, health care and education as key after spending the day in Ethiopia's Somali region, one of the worst-affected areas of the country.

The UN under-secretary-general for humanitarian affairs and emergency relief coordinator emphasized the need for improved security in drought-affected regions so relief agencies can reach vulnerable populations.

She met with Ethiopia's Deputy Prime Minister Hailemariam Desalegn to draw up a plan to ensure access to the country's most volatile areas. In June, two World Food Program workers were detained for over a month in Somali region, also known as Ogaden.

The UN emergency relief agency reports 3.2 million people in Ethiopia depend on food assistance.

The Horn of Africa is currently facing the most severe drought in 60 years, according to the UN, affecting more than 10 million people.

The relief agency will also attempt to provide emergency aid in Somalia after Shebab Islamists announced they would allow access to humanitarian agencies.

"(That) is something we have to test to see whether we will be able to deliver that urgently needed aid into Somalia itself," Amos said.

The drought in Ethiopia is not only affecting crops, but is forcing pastoralists from their communities in search of water.

"Everyone I met spoke of the lack of water and the impact it is having on their day-to-day lives. Their very way of life is at risk," Amos said.

The UN and other relief agencies are struggling to respond to the crisis in light of funding shortfalls and called on the international community for increased aid.

Ethiopia has received $234.4 million to address the crisis, only 77 percent of the amount required, according to the UN.