"Famine is at the door and we are receiving a final warning," UN official Martin Griffiths told a press conference in the Somali capital Mogadishu.
A food and nutrition report has "concrete indications" that famine will strike the regions of Baidoa and Burhakaba in south-central Somalia between October and December, Griffiths said.
"I've been shocked to my core these past few days by the level of pain and suffering we see so many Somalis enduring," said the head of the UN Office for the Coordination of Humanitarian Affairs (OCHA), who began a visit to the country on Thursday.
"The unprecedented failure of four consecutive rainy seasons, decades of conflict, mass displacement, severe economic issues are pushing many people to... the brink of famine," he said.
"We are in the last moment of the 11th hour to save lives."
Somalia and its neighbours in the Horn of Africa including Ethiopia and Kenya are in the grip of the worst drought in more than 40 years after four failed rainy seasons wiped out livestock and crops.
Humanitarian agencies have been ringing alarm bells for months and say the situation is likely to deteriorate with a likely fifth failed rainy season in the offing.
Griffiths said the situation was worse in Somalia than during the last famine in 2011 when 260,000 people, more than half of them children under the age of six.
The UN's World Food Program (WFP) last month said the number of people at risk of starvation across the Horn had increased to 22 million.
In Somalia alone, the number of people facing crisis hunger levels is 7.8 million, or about half the population, while around a million have fled their homes on a desperate quest for food and water, UN agencies say.
Griffiths described scenes of heart-rending suffering during a visit to Baidoa, describing it as the epicenter of the crisis where he saw "children so malnourished they could barely speak" or cry.
"Somalia is facing unprecedented levels of drought which have particularly hit rural communities, alongside other impacts like conflict, Covid-19, macroeconomic challenges, and a recent desert locust upsurge," the UN's Food and Agriculture Organization (FAO) said in a statement on Friday.
It said people's means to produce food and earn income were "stretched beyond breaking point".
The UN's World Meteorological Organization (WMO) has said the Horn was likely facing a fifth straight failed rainy season over the months of October to December.