Somalia's Prime Minister Hassan Ali Khaire said Friday the president will announce a "state of war" against the al-Shabab terrorist group blamed for the country's deadliest attack.

A military official added that the United States is expected to play a supporting role in the new offensive. The official spoke on condition of anonymity because he was not authorized to speak to reporters.

Somalia's army spokesman Capt. Abdullahi Iman saids the offensive will try to push al-Shabab fighters out of strongholds in the Lower Shabelle and Middle Shabelle regions where many deadly attacks on Somalia's capital were launched.

Meanwhile, Somalia's Information Minister Abdirahman Osman said the death toll has risen to 358 in the country's worst-ever attack.

Osman said 56 people were still missing from Saturday's truck bombing on a busy street in Mogadishu. Another 228 people were wounded.