Some Somalis have fled to Ethiopia for fear their children may be forcibly conscripted by Islamist insurgents, a UN refugee official has told the BBC.



The UNHCR's Alison Oman said some mothers at refugee camps in Ethiopia told her they left Somalia as they had nothing left to buy-off the militants.



Al-Shabab controls most of south and central Somalia, including two large regions worst affected by the famine.



The group banned many aid agencies from its territory two years ago.



An estimated 12 million people in the Horn of Africa have been affected by the region's worst drought in 60 years.



Somalia has been worst hit, with tens of thousands of people fleeing to the capital, Mogadishu, controlled by the weak interim government, or to refugee camps in neighbouring Kenya and Ethiopia.

'Routes blocked'



Ms Osman, the UN refugee agency's senior nutritional specialist for the Horn of Africa, said Somali mothers she had interviewed in refugee camps in south-eastern Ethiopia had told her they left their villages often when their last animal died.



"The animals are their insurance. The animals are the bank accounts of these families," she said."A lot of these rural families have been forced to support al-Shabab militias that go through their areas, either giving them the animals or the sorghum they have.



"A couple of mothers said to me the fear was that if they didn't have the animals to give and grain to give, then al-Shabab might forcibly conscript children."



Last month, rights group Amnesty International accused al-Shabab of systemically recruiting children into its ranks.



It said the methods used by the Islamist group ranged from luring children with promises of mobile phones and money, to abductions and raids on schools.



Ms Oman said she had also heard about cases when families had been given small amounts of food for a child to be conscripted.



She added that the number of Somalis arriving in recent weeks in Ethiopia had fallen dramatically.



"A lot of traditional routes that people were using have been blocked by al-Shabab," she told the BBC.



This trapped people in the famine zones where many aid agencies are prevented from supplying the most needy."A lot of internally displaced people who are unable to cross into Ethiopia have said they don't have free passage and it's not safe for them so they're staying put or opting to cross into Kenya," she said.



Meanwhile, in the Ethiopian camps, officials began a mass measles vaccination campaign of children.



The outbreak in the region is proving more deadly because of the high number of weak and malnourished children.



Earlier, the UN special envoy for Somalia, Augustine Mahiga, told the UN Security Council that the UN-backed African Union force in Mogadishu desperately needed extra resources.



Al-Shabab announced it was withdrawing most of its forces from the city over the weekend.



"Without the immediate action to fill this gap, a real danger exists that the warlords and their militia groups will move forward to fill the vacuum created by al-Shabab's departure," AFP news agency quotes Mr Mahiga as saying.



Last year, the Security Council approved a 12,000-strong AU force for Somalia, although the AU said it needed 20,000 troops - and so far it has just 9,000 soldiers on the ground.



Somalia has been wracked by conflict for the last 20 years since the fall of Siad Barre's government.