The pope on Sunday urged the world not to be "indifferent" to the Horn of Africa famine, as the African Union prepares to host a donors conference for victims on August 9.

Pope Benedict XVI spoke of the suffering of millions in the region hit by its worst drought in decades in an address to hundreds of pilgrims at his summer residence in Castel Gandolfo, just outside Rome.

"We must not be indifferent to the tragedy of the hungry and the thirsty," the pontiff said following the weekly Angelus prayer.

"Many brothers and sisters in the Horn of Africa are suffering these days from the dramatic consequences of the famine, aggravated by war and the lack of stable institutions," he said, calling for "compassion" and "fraternal solidarity".

The United Nations has declared a famine in two regions of southern Somalia but the effects of the drought have been felt more widely across the war-torn country, as well as in parts of Djibouti, Ethiopia, Kenya and Uganda.

"It is an immense task. In this time of holiday, let us not forget to open our hands and our hearts to come to the aid of those who need it," the pope said.

"Let us give food and share our bread with the needy," he added.

The deputy chair of the African Union meanwhile said the body will host a donors conference bringing together African heads of state, members of regional economic blocs and international organisations in Addis Ababa on August 9.

"I ask the African continent... to look hard at how they can contribute to alleviating the suffering," Erastus Mwencha said in a statement released Friday.

"Around the globe, everyone must dig deep into their pockets to rescue the people of Somalia from the abyss they find themselves staring into," he added during a one-day visit to Somalia's war-torn capital Mogadishu.

The AU has donated $500,000 to address the crisis in the Horn of Africa but the United Nations said on Friday a total of $2.48 billion was required in order to reach the 12.4 million people affected.

Ongoing violence between Somalia's Islamist al-Shebab militants and pro-government troops has exacerbated the food crisis.

The UN's World Food Programme (WFP) started an airlift of food aid into the capital Mogadishu last week despite battles in the city.

Relief efforts have been hampered by the combat, as well as a ban on some humanitarian agencies by Al-Shebab which controls much of southern Somalia.

The Vatican's official daily, Osservatore Romano, said there was "a race against time" to save the people of Somalia and said the international community should take a more active mediation role between rival Somali clans.

"If international players do not manage to do this, then even a massive humanitarian effort by UN agencies, including the WFP's airlift, and by non-governmental organisations will at best slow the emergency," it said.

US President Barack Obama on Friday called for an international response to avoid a "looming humanitarian crisis in Eastern Africa".

"I think it hasn't got as much attention here in the United States as it deserves," he said after meeting with four African leaders in Washington.

The Shebab rebels have denied there is a famine in Somalia, saying the crisis is being exploited by external enemies.

Shebab spokesman Sheikh Ali Mohamud Rage has claimed that local Muslims are adequately addressing the drought crisis, saying in a speech on rebel radio that there is no need for assistance from "an outside enemy or non-Muslims".

He said the hundreds of thousands of refugees who have been streaming across the Somali border into the mainly Christian countries of Ethiopia and Kenya in search for food were being lured there "so that their faith can be destroyed".

Battered by a relentless civil war since 1991, the plight of Somalis has often been referred to as the world's worst humanitarian crisis. Nearly half of Somalia's estimated 10 million people are believed to be in need of aid.