Four local footballers in Somalia were killed when an explosion believed to have been caused by a bomb ripped through their bus on Friday, police and witnesses said.

Ten players from the Jubaland Chamber of Commerce and Industry club were heading off for a game at the Kismayo city stadium in southern Somalia when the blast hit.

"The players were riding in a bus when the explosion went off, presumably inside the vehicle. There are investigations going on but we presume the device was planted onto the bus," said police officer Mohamed Sadiq.

He said four football players were killed and five others wounded.

"The blast was huge and it caused devastating fire which burned down the bus, people rushed to remove the dead bodies and wounded players," said witness Ibrahim Ahmed.

"I saw the burning bus and three dead bodies of the players -- this was horrible because football players are not politicians," added another witness, Said Adan.

There was no immediate claim of responsibility for the blast in Kismayo, capital of the southern state of Jubaland which borders Kenya.

Jubaland was the first state to start voting this week in long-delayed national elections in Somalia that the Al-Qaeda linked Al-Shabaab jihadist group has threatened to disrupt.

The extremists have been fighting to overthrow the federal government since 2007 and frequently attack government, security and civilian targets.