Humanitarian aid is failing to reach almost two-thirds of those affected by recent fighting in the Somali capital, a key UN official has warned.
The country now represents a worse displacement crisis than Sudan's Darfur region, John Holmes said.
Aid groups estimate 1,300 people have died and 300,000 have fled Mogadishu since fighting flared in March.
Aid workers have accused authorities of hindering the passage of food aid at checkpoints set up across Mogadishu.
"We estimate that we are only reach 35 - 40% of those in need," Mr Holmes said.
"Many are already suffering from a cholera outbreak."
Human rights abuses
Mr Holmes said international law had been violated by the fighting factions in the city, saying that some citizens had disappeared without explanation.
"Clearly, human rights abuses have taken place, but the government categorically denied reports and accusations of their involvement," he said.
Mr Holmes, the most senior UN official to visit the city in more than a decade, had to cut short his trip on Saturday, after bombs exploded in Mogadishu, killing three people.
Somalia has been without an effective national government for 16 years, controlled by rival militias and awash with guns.
An insurgency flared after Ethiopian-backed government forces defeated Islamist fighters in an offensive at the end of last year.
The government now says it controls the city.
Mr Holmes' comments came as European Union foreign ministers called on all the fighting factions to allow the safe passage of humanitarian relief.
In a statement, the ministers urged Somali authorities "to do their utmost to remove any obstacle to the free movement of aid and humanitarian relief workers into and throughout the country."
The government has pledged to remove any hurdles to allow aid convoys to travel unhindered around the city.
Last week, the International Committee of the Red Cross launched a $15m (£7.5m, 11m euros) appeal for the country.
The ICRC said severe floods and droughts had compounded the effects of recent fighting, leaving the population ever more reliant on external aid.