There has been an explosion during a suicide bomb attack close to the port of Somalia's capital, Mogadishu.

"Metal debris fell all over us inside the port and we heard gunfire," a witness is quoted by the Reuters news agency as saying.

"Security forces have surrounded the area."

The blast happened at a checkpoint near the port.

"A suicide bomber tried to strike the police post in front of the port but the security forces shot him and the vehicle exploded," police officer Abdukadir Ahmed is quoted by the AFP news agency as saying.

"Two police officers and five civilians were wounded," he said.

The authorities have not made any formal statement on the number of casualties.

No group has said that they were behind the attack, but the African Union-backed government in Mogadishu has been fighting for control of the country with the Islamist militant group al-Shabab for more than a decade.