The UN says the humanitarian situation in the country has deteriorated rapidly, despite assistance efforts.
It will be the first time that the region has seen famine in 19 years.
Meanwhile, the UN and US have said aid agencies need further safety guarantees from armed groups in Somalia to allow staff to reach those in need.
Al-Shabab, an al-Qaeda affiliated groups which controls large swathes of south and central Somalia, had imposed a ban on foreign aid agencies in its territories in 2009, but has recently allowed limited access.
An estimated 10m people have been affected in east Africa by the worst drought in more than half a century. Tens of thousands of desperate Somalis have been trying to flee their country to neighbouring Kenya or Ethiopia.
The BBC’s Africa correspondent Andrew Harding says the UN is expected to announce a famine in at least two regions in the centre of the country – Bakool and Lower Shabele – where drought, conflict and poverty have now combined to produce the necessary conditions for famine.
Once we have the assurances of security and the ability to have full access to deliver and distribute and monitor, then we will be prepared to go back in
Those conditions include more than 30% of children being acutely malnourished, and four children out of every 10,000 dying daily.
Our correspondent says the emotive word “famine” is used rarely and carefully by humanitarian organisations, and it will be the first time since 1992 that the word has been applied to a situation in Somalia.
Meanwhile, the UN is calling for unhindered access to affected areas, saying that the security situation is hampering humanitarian efforts.
Adrian Edwards, a spokesman for the UN refugee agency, told reporters that the situation for aid workers in Somalia is “not what we want it to be”.
“We do have a very minimal presence, and we have regular visits into the country, but we need significantly better access than we have at the moment to address an emergency of this scale,” he said, speaking from Geneva.
The World Food Programme, which is trying to feed 1.5m people, estimates that as many as 1 million people are in areas it cannot currently access.
Definition of Famine
- More than 30% of children must be suffering from acute malnutrition
- Two adults or four children must be dying of hunger each day for every group of 10,000 people
- The population must have access to far below 2,100 kilocalories of food per day
“Once we have the assurances of security and the ability to have full access to deliver and distribute and monitor, then we will be prepared to go back in,” Emilia Casella, a spokeswoman for the WFP, told the Associated Press news agency.
Johnnie Carson, the US assistant secretary of state for African affairs, says the US was assessing if they were seeing “real change” from al-Shabab, or whether the group planned to impose some kind of “taxation” on aid deliveries.
“Al-Shabab’s activities have clearly made the current situation much worse,” Mr Carson said.
“We call on all of those in south-central Somalia who have it within their authority to allow refugee groups and organisations to operate there to do so,” he said.
‘More resources for piracy’
In a separate development, Amnesty International has said it has found that children in Somalia are being systematically recruited as child soldiers by militant groups such as al-Shabab.
The long-running conflict between the weak, Western-backed government and Islamist extremists is one of the reasons given for fleeing by many of the Somalis who have sought refuge in neighbouring Kenya and Ethiopia.
Drawing on interviews from over 200 Somalis who have fled their country, the rights group says the majority of those recruited are aged between 10 and 17, while some are as young as eight.
The group says children have been used on the front line to fight but also in other roles such as flogging women and girls to force them to follow al-Shabab’s strict rules regarding dress.
The report argues that the failure to tackle such violations of international law contrasts sharply with the resources devoted to the fight against piracy off the country’s coastline.