Twin car bombs that went off in Somalia's capital have killed 18 people and wounded 20 others, the capital's ambulance services said.

"So far we carried 18 dead people and 20 others injured from the blasts tonight," Abdikadir Abdirahman, the director of Amin ambulances, told Reuters.

Police and a Reuters witness said that a car bomb went off in Mogadishu, followed by a second blast and gunfire near the president's residence.

"We understand it is a suicide car bomb near the presidential palace area. Then another blast followed, with gunfire. It is too early to know details and target," Major Mohamed Ahmed, a police officer told Reuters.

A Reuters witness said a huge cloud of smoke could be seen in the sky.