A suicide bomber walked into the office of Mogadishu's mayor and detonated explosives strapped to his waist, killing several people and badly wounding the mayor, Somali police said Wednesday.

The attack occurred shortly after the new United Nations envoy to Somalia, James Swan, had paid the mayor a "courtesy call" and left the compound, an official at the mayor's office told The Associated Press. The official spoke on condition of anonymity because they were not authorized to speak to the media.

The U.N. mission in Somalia in a tweet before the bombing posted photos of the smiling mayor and new envoy, saying Swan had received an overview of the "challenges" in the region.

The mayor, Abdirahman Omar Osman, and his deputy were rushed to a hospital with critical wounds and two district commissioners were among the dead, said police Capt. Mohamed Hussein.

It was not clear how the bomber managed to enter the mayor's office during a security meeting.

The al-Qaida-linked al-Shabab extremist group often targets government buildings such as the presidential palace and other high-profile targets in Mogadishu with bombings.

The Somalia-based group was chased out of Mogadishu years ago but still controls parts of the Horn of Africa nation's south and central regions and is a frequent target of U.S. airstrikes.