Sunday's powerful blast was heard across the city and left at least five vehicles destroyed, according to witnesses.

Several news outlets carried unverified claims that the bombing was the work of al-Shebab militants, whose leader was killed during a US air strike in September.

After Sunday's blast, interior ministry spokesman Mohammed Yusuf said the authorities had had "information" about the explosives-laden car and that it was being pursued at the time of the blast.

"We [had] been following it… but it detonated and four civilians were killed, and the bomber," Yusuf said.

Several news agencies quoted police and intelligence officials as saying that the bomber had driven his car into the convoy of a Somali anti-terrorism unit on the main highway to Mogadishu's airport.

Security officials and civilian pedestrians were among the casualties, they said.

Pro-militant radio station Andalus said a senior al-Shabab member, Sheikh Mohamed Abdu Abdalla, had claimed responsibility for the incident.

Witnesses said security forces opened fire to disperse approaching onlookers after the blast.

Earlier air strike

Early this week, the Somali government said al-Shabab's intelligence chief, Abdishakur Tahlil, had been killed in an another air strike by the United States on Monday night.

Fragile government

Al-Shebab are Al-Qaeda's main African affiliate and are battling to topple the country's fragile internationally-backed government.

The militants were driven from fixed positions in Mogadishu in 2011.

In recent offensives by Somali troops and the African Union's 22,000-strong force known as AMISOM, the rebels also lost several strongholds in the coastal south and center of Somalia.

The group, however, still controls vast rural areas from which militants launch regular attacks against the AU and Somali government troops. They have also struck repeatedly in neighboring Kenya.

Somalia's government is struggling to rebuild the country after decades of conflict sparked of by the 1991 ouster of dictator Siad Barre.

ipj/pfd (dpa, AFP, AP)