* United Nations set to make announcement on Wednesday

* New food security data will be basis for declaration

* Signal to rebels that people's suffering taken seriously  

A malnourished child rests inside the pediatric ward at the Banadir hospital in Somalia's capital Mogadishu, July 19, 2011. [Photo/Agencies]

GENEVA - The United Nations is set to declare famine in parts of southern Somalia, aid officials said on Tuesday, signalling to donors the need for more aid and to insurgents that the population's suffering is taken seriously.  

Mark Bowden, humanitarian coordinator for Somalia, is expected to make the announcement on Wednesday in Nairobi, based on fresh data from the food security and nutrition analysis unit for the violent Horn of Africa country, they said.    

"It will declare famine in several areas of southern Somalia," a Geneva-based aid worker told Reuters, one of several to confirm the expected move.    

The world body has described the Horn of Africa drought as an emergency, one level short of a famine, citing dire levels of acute malnutrition among Somali children reaching camps in Kenya and Ethiopia.    

In all, more than 10 million people are affected and need emergency help, including 2.85 million in Somalia, where one in three children is suffering from malnutrition, the UN says.

Famine is defined as a crude mortality rate of more than 2 people per 10,000 per day and wasting rates of above 30 percent in children under five years old across an entire region, according to the UN Children's Fund (UNICEF).

The UN refugee agency, the UNHCR, said on Tuesday it was seeking further security guarantees from armed rebels in Somalia in order to deliver greater amounts of assistance and prevent more hungry people from becoming refugees.

Al Shabaab, Islamist insurgents affiliated to al Qaeda, control pockets of the capital Mogadishu and swathes of southern and central Somalia.  

Some analysts say the insurgents are allowing aid in fear of a public backlash if they do not. Others say the rebels want to receive bribes.          

RISK OF STARVATION    

At least 500,000 children are at risk of death in the Horn of Africa, where high food prices and the driest years in decades have pushed many poor families into desperate need, UNICEF has said.    

One in 10 children in parts of Somalia is at risk of starving to death, the International Committee of the Red Cross (ICRC) said last week.      

The independent aid agency, one of very few with access to Somalia's worst-hit areas, said that even in the Bay and Lower Shabelle regions, Somalia's traditional breadbaskets, nearly 11 percent of children under five had severe acute malnutrition.

UNHCR has stepped up its work in southern Somalia, distributing aid to 90,000 people in recent days to areas including Mogadishu with another 126,000 due to receive supplies on Tuesday, spokesman Adrian Edwards told a media briefing.

"We need significantly better access than we have at the moment to address an emergency of this scale," he said. "We need guarantees of safety on the one hand, the assurances that the humanitarian nature of our work is going to be respected."    

Al Shabaab surprised aid workers two weeks ago with a pledge to allow relief agencies "with no hidden agendas" greater access to rebel-held territory.    

The World Food Programme (WFP) suspended its aid operations across much of southern and central Somalia in early 2010 after al Shabaab ordered the UN agency to halt operations in areas under its control. The WFP is seeking security guarantees to access these areas and the ability to distribute and monitor aid there, spokeswoman Emilia Casella told reporters.    

The United Nations said on Monday it had started airlifting food to rebel-held areas and that al Shabaab had abided by a pledge to allow relief workers free access.    

"What we believe, and what we have observed, is with the massive movement of population outside of the country some of the fighting forces have realised that they needed to allow humanitarian assistance to come in,"  Raouf Mazou, UNHCR deputy director for the Horn of Africa told reporters. "For how long that will last is something that we don't know."