Somalian Islamist fighters launched a surprise attack and briefly captured a large town today, killing seven, as 40 aid agencies urged the world to help avert a "catastrophic" humanitarian situation.

Rocket-propelled grenades and machine guns were used to seize control of Jowhar, 55 miles north of Mogadishu, the largest of several towns captured in recent months from the western-backed interim government.

"Seven people, including a woman who was a government soldier and her small child that she was carrying on her back, were killed early in the morning when Islamic Courts fighters seized the town," said resident Abdi Ali Osman.

After releasing prisoners in the town, the Islamic group withdrew from Jowhar later in the morning.

Abdirahin Issa Addow, a spokesman for the group, said the aim of the attack was not to take the town, "but to punish our enemy."

The attack highlights the government's inability to assert its authority on Somalia despite support from Ethiopian and African Union troops.

A 15-month Islamist-led insurgency, which has killed more than 6,500 people, has seen a resurgence in recent months.

Agencies said that hundreds of thousands of people are suffering from war, drought and food shortages.

Four smaller towns and a military checkpoint near the capital, Mogadishu, had already been taken before the capture of Jowhar.

Jowhar was the temporary base for the interim government in 2005.

A spokesman for the Islamic Courts, Abdirahin Isse Addow, said four government soldiers were killed in the fighting.

"Our troops entered Jowhar at six in the morning. Few government troops fought us and we defeated them, forcing them to run away," he said.

Somalia has had no effective government since warlords overthrew dictator Mohamed Siad Barre in 1991 and then turned on each other, plunging the country into chaos.

Today's statement by aid agencies, issued by Oxfam, said Somalia now had 1 million internal refugees, their numbers swelling by 20,000 a month, from those fleeing Islamist-government fighting in Mogadishu.

The United Nations children's agency said Somalia is the worst place in the world for children and estimates that one in seven children under the age of five are acutely malnourished.