SOMALIA'S president is expected to announce a "state of war" against the al-Shabab extremist group after last week's truck bombing which killed 358 people in Mogadishu.

The United States will play a supporting role in the new offensive planned by President Mohamed Abdullahi Mohamed, a Somali military official said.

The president's emergency speech has been postponed after the death toll of Somalia's deadliest attack rose to 358, while dozens remain missing.

Somalia's army spokesman Captain Abdullahi Iman said the offensive involving thousands of troops will try to push al-Shabab fighters out of their strongholds in the Lower Shabelle and Middle Shabelle regions where many deadly attacks on Somalia's capital, Mogadishu, and on Somali and African Union bases have been launched.

The extremist group has not commented on the truck bombing last Saturday, which Somali intelligence officials have said was meant to target the city's heavily fortified international airport where many countries have their embassies.

The massive bomb, which security officials said weighed between 600 and 800 kilograms (1,300lbs and 1,700lbs), was instead detonated in a crowded street after soldiers opened fire and flattened one of the truck's tires.

Somalia's information minister Abdirahman Osman said 56 people were still missing. Another 228 people were wounded, and 122 had been airlifted for treatment in Turkey, Sudan and Kenya.