War-torn Somalia is sliding back into an acute hunger crisis with parts of the capital facing emergency levels just short of famine, the United Nations warned Monday.

"Somalia's food security crisis is expected to worsen over the next several months following poor performance of the major rainy season, shrinking humanitarian assistance and access, increasing malnutrition, conflict and surging food prices," the UN's Food Security and Nutrition Analysis Unit (FSNAU) said Monday.

The warning comes three years since over 250,000 people, half of them children, died in a devastating famine.

In Mogadishu, the UN said the crisis is expected to spiral into "emergency" phase, just one step short of famine on its classification scale of hunger.

Thousands displaced by war live in basic makeshift shelters in Mogadishu, where Al-Qaeda-linked Shebab insurgents continue to launch regular attacks against the fragile internationally-backed government.

Among those displaced civilians, "rates of acute malnutrition and mortality levels have surpassed emergency thresholds", the UN said, adding that countrywide, over 200,000 acutely malnourished children under the age of five "require urgent treatment".

Southern and central Somali regions of Bakol, Gedo, Middle Juba, Hiran and Lower Shabelle are also badly hit, it added.

Prices of grain have already soared by up to 60 percent since March in some areas, FSNAU added.

Somalia was the hardest hit by extreme drought in 2011 that affected over 13 million people across the Horn of Africa, with famine zones declared in large parts of the war-ravaged south.

Hardline Shebab insurgents once controlled most of southern and central Somalia, including large parts of the capital, but have been driven out of fixed positions in Mogadishu and Somalia's major towns by a 22,000-strong African Union force.

AU troops launched a fresh offensive in March against Shebab bases. Although they seized a series of towns, the insurgents are thought to have fled in advance and suffered few casualties.