Somalia's al-Shabab Islamists have denied lifting their ban on some Western aid agencies and say UN reports of famine are "sheer propaganda".
The UN on Wednesday said that parts of Somalia were suffering a famine after the worst drought in 60 years.
A spokesman for al-Shabab, which has ties to al-Qaeda and controls much of the country, accused the banned groups of being political.
But the UN insists famine exists and it will continue its aid efforts.
Most Western aid agencies quit Somalia in 2009 following al-Shabab's threats, though some say they have managed to continue operating through local partners.
Some 10 million people are said to need food aid across East Africa but Somalia is by far the worst-affected country, as there is no national government to co-ordinate aid after two decades of fighting.
Thousands of people are fleeing areas under al-Shabab control to camps set up in areas of the capital controlled by the weak interim government, which is battling the Islamist insurgents.
The UN World Food Programme (WFP) was one of those banned.
It says it is planning to airlift food into the capital, Mogadishu, in the coming days to help the thousands of malnourished children who face starvation in the country.
"We are absolutely adamant that there are famine conditions in two regions of south Somalia," WFP's Africa spokesman David Orr told the BBC.
"We've seen the evidence of the emergency in the faces and wasted limbs of the malnourished children who are being forced to trek out of the famine zone, sometimes for days and for weeks."
WFP spokesperson Emilia Casella said the agency would continue to operate where it was possible to do so.
"Al-Shabab is not a monolithic organisation. It's important to note that we're working where we can; we're making plans to work where it's feasible," she told AFP news agency.
The UN's children's agency Unicef said it was increasingly relying on its local partners but had been able to continue its operations.
"At the moment what we are trying to do is to look at how we can scale up our programmes and get more supplementary and therapeutic food into these area where we have the extreme starvation going on," spokeswoman Shantha Bloeman told the BBC.
"Yes, WFP had serious restrictions on it but as far as our operations are concerned we have been operating throughout."
The two districts where a famine has been declared - Bakool and Lower Shabelle - are under al-Shabab control and aid agencies have been wary of resuming activities there amid fears for the safety of their staff.
Al-Shabab spokesman Ali Mohamud Rage earlier this month announced that aid agencies, whether Muslim or non-Muslim, would be allowed back into Somalia as long as they had "no hidden agenda".
This had prompted the US to say it was lifting its ban on allowing its food relief into areas controlled by al-Shabab, which it calls a terrorist group.
However, Mr Rage told journalists in Mogadishu on Thursday night: "The agencies we banned are still banned. The agencies were involved in political activities."
He admitted there was a drought but said reports of a famine were "utter nonsense, 100% baseless and sheer propaganda.
"There is drought in Somalia and shortage of rain but it is not as bad as they put it."
Rashid Abdi, a Somalia analyst at the International Crisis Group think-tank, told Reuters news agency that al-Shabab were trying to avoid being "seen as people who oversaw a large-scale humanitarian disaster".
BBC Somali editor Yusuf Garaad Omar says food aid - as supplied by WFP - is sensitive for al-Shabab as it would like to ensure it goes to regions it controls, rather than government areas.
He also says that a lot of money can be made from transporting food. Helping women and children - the work Unicef does - is less of an issue, he says.
Mr Orr said the situation had been made much worse for many people by the restrictions on access for aid agencies.
"We are appealing for access as humanitarian aid workers. People, as I have said, are starving to death in there. This is a life and death situation," he told the BBC.
"We wouldn't be in this situation had the humanitarian community had access.
"We are appealing to all parties who have an interest in this situation to allow us to go in there and to get the aid in, in as fast and efficient a manner as possible."
More than 166,000 desperate Somalis are estimated to have fled their country to neighbouring Kenya or Ethiopia in recent months.
On Wednesday, UN Secretary General Ban Ki-moon said $300m (£184m) was needed in the next two months to provide an adequate response to the areas affected by famine.