Mogadishu - Thousands of anguished Somalis gathered to pray on Friday at the site of the country's deadliest attack, while the toll rose to 358 and dozens remained missing. Somalia's president will announce a "state of war" against the al-Shabaab extremist group blamed for the bombing, the prime minister said.The United States is expected to play a supporting role in the new offensive that President Mohamed Abdullahi Mohamed is set to launch on Saturday, a Somali military official told The Associated Press. The official spoke on condition of anonymity because he was not authorized to speak to reporters.Somalia's army spokesman Capt. Abdullahi Iman said the offensive involving thousands of troops will try to push al-Shabaab 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.Also Friday, the US military said it had resumed its fight against al-Shabaab with a drone strike.The extremist group has not commented on Saturday's truck bombing in Mogadishu, 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 600kg and 800kg, instead detonated in a crowded street after soldiers opened fire and flattened one of the truck's tires.New offensive Somalia's information minister Abdirahman Osman said late Friday that 56 were people still missing. Another 228 people were wounded, and 122 had been airlifted for treatment in Turkey, Sudan and Kenya."This pain will last for years," said a sheikh leading the Friday prayers at the bombing site, as long lines of mourners stood in front of flattened or tangled buildings.Since the election of the country's Somali-American president in February, the government has announced a number of military offensives against al-Shabaab, Africa's deadliest Islamic extremist group, only to end them weeks later with no explanation. Experts believe that has given the extremists breathing space and emboldened them in their guerrilla attacks.Iman, Somalia's army spokesman, told the AP that troops recaptured three towns in Lower Shabelle region from al-Shabaab on Friday in preparation for the new offensive.Somali officials did not give details on what role the US military might play. There was no immediate response late Friday from the US Africa Command.Logistic training The US has stepped up military involvement in the long-fractured Horn of Africa nation since President Donald Trump approved expanded operations against the group early this year. The U.S. has carried out at least 19 drone strikes in Somalia since January, according to The Bureau of Investigative Journalism.The latest US drone strike occurred Monday about 35 miles (56 kilometers) southwest of the capital, the US Africa Command told the AP earlier Friday. It said it was still assessing the results.Earlier this week, in response to questions about the massive truck bombing, a Pentagon spokesman said the United States has about 400 troops in Somalia and "we're not going to speculate" about sending more.In April, the US announced it was sending dozens of regular troops to Somalia in the largest such deployment to the country in roughly two decades. The US said it was for logistics training of Somalia's army and that about 40 troops were taking part.Weeks later, a service member was killed during an operation against al-Shabaab. He was the first American to die in combat in Somalia since 1993.