Several residents in the airport area said the shelling started around five minutes after a military plane apparently transporting goods for African Union peacekeepers landed.
AU forces responded by firing their own mortar shells, witnesses said.
An AFP correspondent in the area said he heard at least eight mortar explosions but could not immediately establish if there were any casualties.
The Associated Press said workers managed to safely offload the cargo, quoting Eydurus Sheik, an immigration officer at the airport.
Somalia’s Al-Shebab movement earlier this week warned that all flights should cease as of September 16, arguing that the airport was an instrument of Ethiopia’s military occupation of Somalia.
Commercial activity at the airport has since stopped, despite government assurances that the radical Islamist militia did not have the means to impose a blockade on the airport.
The airport is used for both commercial and military flights but is also the main base for the Ugandan contingent of the African Union peacekeepers, who were reinforced by Burundians earlier in 2008.
With the war-torn Somalia’s roads dotted with rogue checkpoints and freelance gunmen and its waters infested with pirates, traders have warned the airport’s closure would only further stifle an already agonising nation.
