Islamist forces battling the country’s transitional government briefly took over a police station and other key buildings in Karan district.
Thousands are fleeing the area, previously a refuge for the displaced.
“The government is weakened by the rebel forces,” AFP news agency quoted Sheikh Aden Mohamed Nur as saying.
“We ask neighbouring countries – including Kenya, Djibouti, Ethiopia and Yemen – to send troops to Somalia within 24 hours.”
Somalia has been without an effective government since 1991. Its UN-backed, transitional government controls only parts of Mogadishu, and little of the rest of the country.
Several thousand Ethiopian troops left Somalia in January after a two-year intervention in support of the transitional government.
There are some 4,300 African Union troops deployed in Mogadishu, but they lack any mandate to pursue the insurgents.
Reuters news agency quoted a spokesman for militant Islamist group al-Shabab as warning Kenya not to intervene.
“If it tries to, we will attack Kenya and destroy the tall buildings of Nairobi,” Sheik Hasan Yacqub told reporters in southern Somalia.
Kenya had said it would not stand by and let the situation in Somalia deteriorate further because it would destabilise the region, Reuters reported.
Pro-government forces have been fighting radical Islamist guerrillas in the capital since 7 May.
Last Friday, gunmen killed Mohamed Hussein Addow, a politician who represented Karan.
It was the third killing of a high-profile public figure in as many days.
Somalia’s security minister – an outspoken critic of the militant Islamist group al-Shabab – was killed in a suicide attack in the northern town of Beledweyne, and Mogadishu’s police commander was also killed this week.
Militant groups including al-Shabab, which is accused of links to al-Qaeda, have been trying to topple Somalia’s government for three years.
A moderate Islamist president took office in Somalia in January but even his introduction of Sharia law to the strongly Muslim country has not appeased the guerrillas.
Some four million people in Somalia – or about one-third of the population – need food aid, according to aid agencies.
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