Somali rebels are blocking men fleeing harsh drought and forcibly recruiting them, while women and children who escape alone risk attack and rape in crowded camps, witnesses and officials said.

"They captured me and beat me," said Nor Ibrahim, who was attacked three times before he finally escaped into neighbouring Kenya.

"They told me they would make into a militia man, and if I refused they said they would kill me," the 20-year old added, who arrived naked at the Dadaab refugee camp in eastern Kenya, after bandits stole his clothes.

Up to 1,300 Somalis stream daily into Dadaab, the world's largest refugee complex, where in July alone some 40,000 Somalis arrived -- the largest influx in 20 years, the UN refugee agency said.

Eighty percent of new arrivals are women and children, the UN children's agency UNICEF said on Friday. Witnesses say many of the men who do arrive are elderly.

Al-Qaeda affiliated Shebab gunmen, who control large swathes of southern and central Somalia, are preventing men of fighting age from escaping one of the worst droughts for decades.

Tens of thousands of people have died from the drought, with the UN declaring famine in three new areas this week in southern Somalia, all in areas the Shebab control or are fighting over.

"The militias are trying to recruit young men," said 80-year-old Adan Ali at the reception centre in Dadaab's Dagahaley camp.

"We were leaving our area...but men came with guns and said we couldn't leave.

"I got out of the car and walked and said to them 'kill me, kill me', and they let me go," said Ali, who walked for several weeks to reach the Kenyan camps.

The Shebab have been accused of exacerbating the drought by banning several key foreign aid groups from their fiefdoms.

US Secretary of State Hillary Clinton on Thursday urged Shebab militants to stop blocking aid to famine-hit areas of Somalia and allow it to reach masses of starving people.

The UN has described drought-hit Somalia as "the most severe humanitarian crisis in the world today and Africa's worst food security crisis since Somalia's 1991-92 famine."

Shebab fighters have waged a bloody campaign to overthrow the country's Western-backed government, with heavy clashes in the Somali capital last week.

"I tried to leave but the militia told me to go back from where I was from," said Suliman Said, aged 35.

"I took a different route but was captured again. They held me for seven days before I accepted what they were saying and went back home," added Said, who later sneaked away at night and made it to Kenya.

In the southern Somali town of Doolow close to the Ethiopian border, where hundreds of displaced people have arrived fleeing drought, the situation is even more stark.

A crowd of women and children wait for food handed out by local aid agencies, with just a handful of men amongst them.

"Less than two percent of the people here are men," said Adan Shiek from Cafdro, a local NGO. "You can see for yourself they are not coming."

Aid agencies in Doolow say it is because the Shebab is refusing to let men cross into areas controlled by the transitional government, as they fear they will then be trained to fight against them.

"My husband had to send me here alone," said Habiba Isaac, a 30-year-old Somali woman who had trekked for nine days to reach Doolow.

"I had to make the journey carrying my children, I thought I'd die from hunger."

Severe drought in the Horn of Africa has left 12 million people facing starvation, and Somalia is the worst affected country in the region.

"Reports of children dying along the way from Somalia or just as they arrive at the camps are disturbingly common," UNICEF said in a statement.

"I left my husband behind," said 40-year-old Habiba Gurow, who fled with her son to Dadaab. "There was a gun fight and we got separated - I don't know if he's still alive," she added.

The journey from Somalia to Kenya can take weeks or even months, and with no men to help them, women must make difficult decisions.

"We've heard stories that some children are left behind so mothers can save the stronger ones," said Ivy Ndungu of Save the Children. "It has a traumatic effect on the women who are coming here."

With swelling refugee numbers, the living conditions in the camps are dire; with a lack of shelter, adequate food and medical care.

"In the camps where you have severe overcrowding, sexual and gender based violence is a real concern, particularly for women who are here on their own, or with young children," said Chris Tidey of UNICEF.

"Single parent families headed by women are at a heightened risk."