As many as 750,000 people could die as Somalia's drought worsens in the coming months, the UN has warned, declaring a famine in a new area.
The UN says tens of thousands of people have died after what is said to be East Africa's worst drought for 60 years.
Bay becomes the sixth area to be officially declared a famine zone - mostly in parts of southern Somalia controlled by the Islamist al-Shabab.
Some 12 million people across the region need food aid, the UN says.
The situation in the Bay region was worse than anything previously recorded, said senior UN's technical adviser Grainne Moloney.
"The rate of malnutrition [among children] in Bay region is 58%. This is a record rate of acute malnutrition," she told journalists in the Kenyan capital, Nairobi.
This is almost double the rate at which a famine is declared.
"In total, 4 million people are in crisis in Somalia, with 750,000 people at risk of death in the coming four months in the absence of adequate response," the UN's Food Security and Nutrition Analysis Unit (FSNAU) says.
Half of those who have already died are children, it says.
Neighbouring Djibouti, Eritrea, Ethiopia, Kenya and Uganda have also been affected by the severe lack of rain.
But 20 years of fighting and the lack of a national government mean that Somalia is by far the worst affected country.
The UN-backed authority controls the capital, Mogadishu but few other areas.
Unni Karunakara, head of medical charity Medecins Sans Frontieres (MSF), says al-Shabab's restrictions on aid workers mean many people in Somalia cannot be helped - and says aid agencies should be more open about this when appealing for more money.
"The grim reality of Somalia today is we are not able to get to south and central Somalia, which we consider to be the epicentre of the crisis," he told the BBC World Service.
"What is needed is a better representation of the challenges that aid agencies, including MSF, face in delivering assistance in Somalia today.
"Even if we are able to get food and supplies to the main ports of Somalia, I think there is a real challenge in being able to deliver that assistance - what I call the 'last-mile' problem.
Some officials from al-Shabab, which has links to al-Qaeda, have accused Western aid groups of exaggerating the scale of the crisis for political reasons.
Tens of thousands of Somalis have fled their country to seek help.
BBC East Africa correspondent Will Ross says that even if there is rainfall in October or November, people will need food aid for several more months until the crops have grown.
"This isn't a short-term crisis," said UN Humanitarian Coordinator for Somalia Mark Bowden.
In Kenya's Wajir district, just across the border from Somalia, health workers are reporting an increase in the number of malnourished children.
Weakened by the lack of food they are more susceptible to disease.
The drought is still taking its toll on the livestock, says our correspondent, and people living in the arid areas of Kenya depend on their animals for their livelihood.
With no rain expected for several weeks the crisis is still deepening despite the presence of aid agencies.