A landmine blast in the Somali capital Mogadishu has killed eight people, and mortar rounds fired by insurgents at the city airport as the president returned have killed a further six.

Al Shabaab Islamist fighters fired the mortar rounds shortly after President Sheikh Sharif Ahmed and the parliament speaker landed late on Saturday. Government troops and African Union (AU) peacekeepers responded by shelling rebel strongholds.

"All the mortar rounds missed the airport but landed in civilian areas," Ali Muse, coordinator of ambulance services, told Reuters.

He said five of the six people killed died in the bustling Bakara market from where al Shabaab often launches its attacks. Four of them were women.

On Tuesday, the United Nations urged Somali security forces, AU troops and Islamist militants not to indiscriminately shell densely populated areas, saying this was a blatant violation of the laws of war.

Mired in violence and awash with weapons since the ousting of a dictator in 1991, Somalia has lacked an effective government for almost two decades.

Residents in the battle-scarred capital's Waberi neighbourhood said eight people died when a landmine buried near a tea shop popular with government soldiers exploded late on Saturday in an apparent attack on the security forces.

"The explosion killed five soldiers and three (civilians). Human flesh was everywhere and some of the injured victims were screaming for help," eyewitness Yusuf Abdulqader said.

No group has claimed responsibility for the attack but bitter relatives of the dead pointed the finger at al Shabaab.

"They have massacred our people, they don't care for our people, our flag and sovereignty," wept Fadumo Abdi, whose son was killed in the blast.

Somalia's parliament had been expected to convene on Sunday for the first time since December after repeated setbacks, but the meeting was postponed for four days for "technical reasons".

Scores of Somali legislators have fled violence at home to the safety of countries in Africa and Europe and to the United States, leaving parliament without a quorum to meet.

- Reuters