Trapped in famine-hit Afgoye, a rag-hut city ruled by Islamist Shebab rebels and the world's largest camp for displaced people, Saedo Saleh knew she had to escape when her baby son fell sick.

"There is nothing there, no aid of any kind," she said, cradling her screaming one-year old son in a clinic in the government-held but war-torn Somali capital, where she sought help after sneaking across the frontline.

"People are struggling, because there is not enough food, people are sick and there is nothing for them there," said Saleh, whose son received treatment for his fever, as well as injections against measles.

Kenyan troops and tanks crossed the border into southern Somalia at the weekend to assault hardline Shebab positions there, after a spate of kidnappings of foreigners inside Kenya, but the rebels remain in control of much of southern and central Somalia.

Afgoye, a crowded corridor of some 410,000 people along a key road running west out of Mogadishu is struggling under the Al-Qaeda linked Shebab's draconian restrictions on foreign aid.

Almost three months since the UN declared famine in the first of several southern Somali regions -- now including Afgoye and camps inside Mogadishu -- the situation remains grim, despite international efforts to increase aid.

The United Nations has described Somalia, where a civil war has been going on since 1991, as facing the most severe humanitarian crisis in the world.

Four million Somalis are in crisis, with some 750,000 at risk of dying in coming months if efforts are not scaled up, the UN Office for the Coordination of Humanitarian Affairs (OCHA) has warned.

Even inside Mogadishu, base of a 9,000-strong African Union force and the weak Western-backed government, recent fighting and security concerns have limited access to desperate people needing food, shelter and medicine.

"We are doing are best, but the demands are enormous," said Ibrahim Mohamed Kasim, manager of the mother and child Hanano clinic -- meaning "care" in Somali -- in central Mogadishu.

"The aid has increased but it is not being coordinated well; people are still in need and more people are arriving," he added.

Heavy seasonal rains have begun, providing some relief for cattle herders, but adding to the misery of the tens of thousands living in rag and plastic shelters, having fled war and drought in their home areas.

"Infectious diseases -- including cholera, pneumonia, dengue fever and malaria -- are common in the city" with the rainy season feared likely to increase their spread, Doctors Without Borders said in recent statement.

"There is not enough food, and my family is getting sick," said Rahma Abdullah, a former shepherd from Baidoa region, who came to the capital after his animals died, and lives in a plastic shelter in a makeshift camp.

"The (government) soldiers here now are better than the Shebab, but they are not strong, they ask us for food and money."

The Shebab, who have vowed to step up a bombing campaign after killing at least 82 people in a suicide attack in Mogadishu earlier this month, have been pushed out of most of their positions in the capital but remain a threat.

"Security is better here in Mogadishu these days, but I think the Shebab will launch more bomb attacks," said Sara Ibrahim, a mother of six, living in basic plastic shelter provided by a Qatari aid agency.

"Some have come to help us, but we need more, because we have so little -- the food we get is just enough to survive from day-to-day, but not enough to live a life."

Children weakened by hunger are now "sitting out in the rain all night, and risking contracting diseases," said Sonia Zambakides, from Save the Children Somalia office, adding that two children were recently swept away by flood waters.

"The rains are going to get worse, and we must provide help to these families as this crisis enters a new phase," she said.

Food deliveries have reached some 2.2 million people as of last week, over half of those in need and massive increase since a few weeks ago, OCHA's latest report reads.

But getting aid out across the areas of southern Somalia hardest hit by "constant security threats and access restrictions," is tough OCHA has warned, adding that Shebab fighters seized ten food aid trucks earlier this month.