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.

On Sunday, BBC 6 Music DJ Mark Radcliffe explores the history of the football chant to find out what makes a hit in the stands.
Before the programme goes live, we want to test your chant knowledge. Listen to these seven terrace chants and work out which club's fans are singing them.
You can listen to 'From Doris to Depeche: The football fans' songbook' on BBC Radio 5 live from 19:30 BST.

Speaking on RT?? radio, the minister said it would not be possible to get rid of the Health Service Executive structure this year.
He said he wanted to focus on reforms that improve GP practices and building and developing hospital groups.
Mr Varadkar said the 500m euros overrun in healthcare spending was due mainly to a significant increase in the number of people using services this year.
He also said he could not put a deadline on when free GP care for children under six would be available.
When asked if it would be in place as promised by the end of this year, he said: "I can't say that for certain.
"That depends on how the talks and negotiations go with the IMO (Irish Medical Organisation). It is very important to me that we bring GPs with us.
"This is a huge opportunity to put resources back into general practice, I hope they see that, and as well as that, it's an opportunity to change the way we provide healthcare."
Mr Varadkar said he hoped to have a report from an expert group by the end of this month looking at whether medical cards should be issued to people on the basis of their medical condition.
The minister said he would have to see what the group proposed, whether it was affordable and then discuss it in the context of the budget.
He also said the issue of tax relief on health insurance was an issue for Finance Minister Michael Noonan.
Nearly five million people in Somalia are suffering from a shortage of food due to poor rainfall, floods and displacement, the United Nations says.
[NEXT_CONCEPT]
From Blue Moon to You'll Never Walk Alone, almost every team have their own song - but do you recognise some of the lesser-known chants?
[NEXT_CONCEPT]
Irish Health Minister Leo Varadkar has said the timeframe for the reform of the country's health service was overambitious from the start.