More than 300,000 children under the age of five are severely malnourished and require urgent assistance.
Most of those in need of help have been internally displaced following decades of conflict.
Malnutrition levels in Somalia have increased over the last six months with nearly half the population affected.
The number of people without enough food has increased by 300,000 since February.
Somalia drought leaves children 'facing death'
Peter de Clercq, the UN humanitarian coordinator for Somalia, said those providing aid were ready to "scale up" their response to help families struggling to find food.
But he noted that funding for the Somalia Humanitarian Response Plan has reached just 32% of its target.
The crisis follows poor rainfall in the south and central parts of Somalia where crop production has fallen by more than half in just six months. Farmers have also lost many of their livestock in the recent drought.
Aid agencies have warned that the numbers could go higher if the Dadaab refugee camp in Kenya is closed down and thousands are forced to return to Somalia where they have no homes or livelihoods.
In January, aid agencies launched an appeal for more than $880m (Â£680m) to alleviate the humanitarian situation in Somalia but, so far, less than half of that sum has been donated.
The UN report, released on Tuesday, suggests that crop production was severely hit by insufficient rainfall and flash flooding in southern and central Somalia, "the breadbasket of the country".
The agency said continued population displacement following decades of conflict and attacks by extremist groups such as al-Shabab was a major contributing factor.
Who are Somalia's al-Shabab?
Al-Shabab, the Islamist militant group, has been battling the UN-backed government in Somalia for years and has carried out a string of attacks in neighbouring Kenya.
The group, which is allied to al-Qaeda, has been pushed out of most of the main towns it once controlled, but it remains a potent threat.
Nearly five million people in Somalia are suffering from a shortage of food due to poor rainfall, floods and displacement, the United Nations says.