Witnesses in Somalia's capital say an exchange of mortar fire has killed at least seven people and injured 12 others.

Residents say Wednesday's fighting began when an African Union plane landed at Mogadishu's main airport in defiance of a ban by the Islamist militant group al-Shabab.

The insurgents fired mortars at the airport, triggering a counter-attack. It is not clear whether those shells came from African Union peacekeepers, Ethiopian troops, or Somali government forces.

Al-Shabab warned last month that it would attack any planes landing or taking off from the airport. The group said the flights benefit Ethiopia, which has thousands of troops in Somalia backing the interim government.

The Somali government has urged airlines to use the airport despite the threat.

On at least two other occasions, Islamists fired on the facility after planes successfully landed. 

Some information for this report was provided by AFP.