Somalia's Al-Qaeda-linked rebels said Thursday they were moving over 12,000 starving families back into famine zones they had fled, where the UN has warned they will die without help.

Draconian aid restrictions imposed by the extremist Shebab are blamed for turning harsh drought across the Horn of Africa into famine in the areas they control, with 750,000 people at risk of death in coming months, the United Nations has said.

"The mujahedeen fighters, in their bid to help people displaced by drought, started working on plans to send them back home where they will be assisted, God willing," said Sheik Mahad Abu-Safiya, a senior Shebab official.

The families, estimated to number at least 50,000 people, were "taken back to their homes with packages to feed them for three months," he added.

Witnesses said the packages included rice, maize and cooking oil.

However, the Shebab have refused most international assistance, and blocked people fleeing drought and famine in the Bay and Bakool regions from travelling in search of aid to Mogadishu, where relief efforts are centered.

Crowded trucks began moving people late Wednesday from camps in and around the Shebab-held town of Baidoa back to their original villages, up to 50 kilometres (30 miles) southwest of the town, officials and witnesses said.

"The process has started and we have moved the first of the 12,000 displaced families to their original locations," Mohamed Walid, another Shebab official, told reporters.

"Most of the displaced people were moved from a big camp at Baidoa airport, they were taken on long trucks," said witness Osmail Mohamed.

"I have seen people returning onboard trucks with food distributed to them by the Shebab," said Abdulahi Derow, who said the food they were given was "enough for a month."

Local aid workers said the Shebab had ordered them to help move people back to the villages they had fled from.

"The Shebab group started sending people back to their homes, and gave them some food," said one Somali aid worker said, asking not to be named. "But people are still in need of help."

Shebab fighters last month pulled out of positions in the war-torn capital Mogadishu where they were battling the weak Western-backed government, but they still control swathes of south and central Somalia.

The United Nations has declared six regions in south Somalia famine zones, the majority in Shebab-controlled areas.

Drought, high food prices and fighting in Somalia have increased the number of those in need of humanitarian assistance across the Horn of Africa to 13.3 million, according to the UN.

Access to Shebab-controlled areas is a major concern, with a group of 20 international and Somali aid agencies calling on Wednesday for talks with the Shebab to create "free passage of assistance."

The agencies warned the situation was the worst they had ever seen in decades of work in Somalia, and was expected to deteriorate further with rains next month likely to worsen conditions for disease.

It is feared that cholera, measles and malaria will have a devastating effect on a people already weakened by harsh drought.

"Never before have we faced such acute suffering with so many lives at stake," they warned in a joint letter. "Somalia is at a turning point."

Ken Menkhaus, professor at Davidson College in the US state of North Carolina, called for a "diplomatic surge" from the "West and the Islamic world" to ensure both the Shebab and the Western-backed government allow access to affected people.

Writing in a paper Thursday for the Washington-based Enough pressure group, Menkhaus called the Shebab an "Islamic Khmer Rouge, in which an armed group with a deeply twisted interpretation of the faith presides over the mass deaths of its own people."

The Shebab "must be made to justify its policy to Islamic leaders and scholars," Menkhaus added.

"How, precisely, does allowing hundreds of thousands of captive fellow Muslims to starve advance any Islamic or Somali cause?"