Al-Qaeda-linked militants have attacked a police base in Somali capital with two car bomb blasts, leaving at least nine people dead, including four attackers.

Al-Shabab gunmen stormed the headquarters of Somalia's Criminal Investigation Department (CID) in Mogadishu after the blasts on Sunday, triggering a fighting which was ongoing.

The assault started with twin car bombings at the gates of the security facility after which at least two gunmen on foot fought their way inside.

Witnesses said heavy gunfire rang out inside the police base for about half an hour after the first blast.

It was the second major attack in the city this week by the militants who have kept up their war on the Western-backed government in the face of US drone strikes and African peacekeeping forces.

The attacks come as Somalia is campaigning for a presidential election due in August.

The country, situated on the Horn of Africa, has been the scene of brutal fighting between al-Shabab and government forces for a decade.

Government troops joined by the African Union Mission in Somalia (AMISOM) forces have driven out the Takfiri terrorists from Mogadishu and other major cities but al-Shabab has continued to carry out sporadic attacks in the capital.

Victims of the attacks have included international aid workers, journalists, civilian leaders and African Union peacekeepers.

In June, an al-Shabab bomber first detonated an explosives-laden car at the gate of a hotel in Mogadishu that was often used by politicians and tourists, then, more militants moved in and killed at least 14 people at the scene.

The al-Shabab militants have also attacked targets in neighboring Kenya, killing people, including officers, and looting government building there.