Radhika Coomaraswamy, a special representative for the United Nations, said famine has forced many Somalis to leave their homes. Many women have lost the protection of family and clan.
"The situation is intensifying," she said.
Al-Shabaab, an Islamist group, sometimes forces women to enter brief marriages that are a cover for sexual slavery.
"For the [al-Shabaab], forced marriage is another aspect they are using to control the population," Coomaraswamy said.
Sheik Mohamed Farah Ali, who left al-Shabaab to fight on the government side, said girls as young as 12 are forced into temporary marriages. If they refuse, they are killed.
"There's no cleric, no ceremony, nothing," he said.
The United Nations said at least 2,500 recent cases of sexual violence have been reported from Mogadishu alone.