Extreme drought is affecting more than 10 million people in the Horn of Africa.
Two regions of southern Somalia: southern Bakool, and Lower Shabelle have been hit, a statement by the UN Office for the Coordination of Humanitarian Affairs for Somalia said.
Both are areas controlled by Al-Qaeda inspired Shebab insurgents.
"Across the country nearly half of the Somali population - 3.7 million people - are now in crisis," the statement read.
Officials warned that unless urgent action was taken the areas afflicted by famine would grow.
Famine implies that at least 20 percent of households face extreme food shortages with limited ability to cope, acute malnutrition in over 30 percent of people, and two deaths per 10,000 people every day, according to the definition released by the UN.
Meanwhile, the UN and US have urged Shebab rebels to provide further safety guarantees to allow aid groups to reach those in need.
Al-Shabab, which controls large parts of south and central Somalia, had imposed a ban on foreign aid agencies in its territories in 2009, but has recently allowed limited access.