Mogadishu - A roadside bomb exploded and killed at least 20 people in a minibus in Somalia's Lower Shabelle region on Thursday evening, the region's governor said, blaming the extremist group al-Shabaab.Most of the victims were women, Ibrahim Aden Ali said. He called the blast a "national disaster."Six other people were injured, regional security official Nur Abdullahi told The Associated Press.The massive bomb buried beside the road struck the vehicle near Golweyn village and largely destroyed it, Abdullahi said."It's a horrible incident. Bodies are lying in a pool of blood, and some of them were burnt beyond recognition," he said.Isaq Haji, a nurse at a hospital in Marka town 90km south of the capital, Mogadishu, said they had received four victims with "horrific" wounds.There was no immediate claim of responsibility.The Somalia-based al-Shabaab claims control over parts of the largely coastal Lower Shabelle region, which has been a focus of efforts to counter the group by a 22 000-strong multinational African Union force.The area where the bomb exploded is a contested one. Roadside bombs are commonly used by the al-Qaeda-linked al-Shabaab.Civilians often have been casualties in this long-chaotic Horn of Africa country. Now hundreds of thousands of Somalis are on the move as a drought threatens roughly half of the country's population of 12 million.New Somali-American President Mohamed Abdullahi Mohamed, who was elected in February, has promised to make security a priority as the weak central government tries to assert itself beyond the capital and some other urban areas.