At least 13 people have been killed in Somalia's capital, Mogadishu, after African Union peacekeepers exchanged mortar fire with fighters from the al-Shabab group.

Members of the group, which is fighting to topple the UN-backed government, fired mortars at the parliament building on Sunday.

Local police said that the artillery fire missed its intended target, but the African Union forces responded by shelling al-Shabab positions in Mogadishu's Bakara market.

The return fire killed 13 civilians and injured dozens more, sources told Al Jazeera.

"Several artillery salvos and mortars hit Bakara market," Ali Muse, the co-ordinator of a Mogadishu ambulance service, said.

It was the first meeting since December for the Somali parliament, many members of which live outside the country because of security fears.

MPs had been scheduled to hold a vote of confidence in the government and a "no" vote could have forced the government to resign.

Sharif Ahmed, the president, has so far failed to deliver on the hope he would be able to unite some of the country's warring factions and establish a greater degree of central power following his election in January 2009.

The Transitional Federal Government controls just a small portion of the capital, while al-Shabab and allied armed groups control much of the rest of the city, as well as large areas of south and central Somalia.

Government forces have repeatedly said that they are planning an offensive against al-Shabab and the other groups in Mogadishu, but they are still to make an significant move against them.