The African Union is due to hold a much delayed summit in the Ethiopian capital Addis Ababa to pledge funds for the famine gripping the Horn of Africa.The AU has already pledged $500m (£300m), but the UN says that at least another $2bn is required to help those in need.At least 12 million people in Somalia and neighbouring countries require emergency assistance, the UN says.Tens of thousands are believed to have died since the crisis began.Response criticisedAfrican Union Commission chief Jean Ping urged Africans to "act out against hunger by providing both cash and in-kind support for urgent life-saving assistance to our brothers and sisters in the Horn of Africa".The US, Britain, China, Japan, Brazil and Turkey have all pledged funds to the region, as has the Organisation of the Islamic Cooperation (OIC) - a gathering of Islamic countries - but the contributions have fallen short of the requested assistance.Correspondents say several African governments have faced criticism for their lack of response, and Aid agency Oxfam says only a handful of African countries have donated money so far.The food crisis is said to be the most serious to affect the continent since the famine in Somalia in 1991-1992.