Witnesses said heavily armed men belonging to al-Shabab, a group alleged by US officials to have connections to al-Qaeda, cordoned off the area, preventing people from reaching the scene.

"We don't know what caused the explosions but the house where it hit has totally collapsed and I saw one burned body ... under the rubble," Adan Yusuf, a witness, said.

Another witness, who spoke on condition of anonymity, said everybody had heard the loud explosions but dozens of al-Shabab fighters had barred access, so it was not possible to know how many casualties there were.

Police arrested two men who had been guarding fighters digging a hole at a bridge in Mogadishu for the roadside bomb and confiscated a bag with materials for making explosives, the government said.

Food aid 'confiscated'

In another development, Somali officials said, al-Shabab has intensified its campaign to stop humanitarian supplies distributed by the World Food Programme (WFP) by issuing tough warnings against people who work with the aid agency.

The group has confiscated large quantities of food aid in the past 24 hours from local traders they accused of being contracted by WFP, the officials said, and also burned some of the food they collected from warehouses in Mogadishu.

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"The WFP wants to poison our people but we will never allow it," Sheik Ali Mohamed Hussein, a senior al-Shabab member in Mogadishu, said.

"Our forces have collected the expired grain from warehouses in the town where local traders stored it and we destroyed it."

Hussein asked people to avoid food with WFP markings or risk punishment.

"We warn all traders and residents in Mogadishu to stay away from WFP foods. Anyone found with WFP market supplies in his possession will face punishment," he said.

Fighters raided nearly 30 warehouses in Beldweyne, a town 400km north of Mogadishu, confiscating food that they claimed belonged to the WFP, sources told the AFP news agency.

They also captured a convoy of  food aid on its way from central Somalia to villages in the region.

Al-Shabab and another group have been fighting Somalia's UN-backed government since the start of 2007, launching frequent attacks on its bases in Mogadishu.

Al-Shabab controls much of the capital in a country deprived of an effective central government for nearly two decades.

More than 21,000 Somalis have been killed in fighting since the start of the uprising, 1.5 million have been uprooted from their homes and nearly half a million are sheltering in other countries in the region.