UNITED NATIONS — The U.N. humanitarian chief warned that terrible atrocities and urban warfare were engulfing Somalia's capital and called for a large increase in aid to handle a burgeoning population of refugees fleeing Mogadishu.

Undersecretary-General John Holmes also painted a grim picture of conditions for millions of people uprooted across the Somali border in Ethiopia's Ogaden province — which faces possible famine — and in Sudan's Darfur region, where violence continues.

In Somalia, he said, the U.N. estimates that more than half of the population of the capital, Mogadishu, — about 600,000 people — have fled an upsurge in violence and intimidation.

Holmes said 230,000 are now living along a 9-mile stretch of road between the capital, Mogadishu, and the town of Afgooye, which probably constitutes the single largest gathering of displaced people in the world today.

He told the U.N. Security Council that he drove along the road where over 70 makeshift camps have mushroomed and met many people who fled with only the clothes on their backs.

He appealed for humanitarian organizations to step up their presence and said at least $400 million will be needed in 2008, up from $300 million this year, to help some 1.5 million people in need. But he stressed that "a robust humanitarian response cannot make up for the absence of desperately needed political and security progress."