Witnesses reported the plane burst into flames and one of its wings fell off before the aircraft crashed. The fate of the 11 people on board is not yet known.
Government and AU sources said the plane was an Ilyushin, one of the largest cargo planes in the world, and was carrying technicians who had been working on another aircraft used by the peacekeepers.
Egi Azarian, the head of Belarus-based company that owned the plane, said he had been informed it was shot down by a missile.
The crash came at the end of a violent week of clashes between Islamic militias and Somali and Ethiopian forces in Mogadishu. At least 20 people were killed and the Red Cross Estimated 300 more were wounded in fighting and mortar attacks.
"This is a tragic situation," the U.N. humanitarian coordinator for Somalia, Eric Laroche, told Reuters. "Tens of thousands of people are fleeing Mogadishu and civilian casualties are mounting daily."
African Union peacekeepers were deployed to the city earlier this month to replace Ethiopian forces and help with security. Only 1,200 of the planned 8,000 AU soldiers have been deployed.
On March 9, an Islamic group claimed it had hit a plane carrying several AU peacekeepers with a missile. That plane was forced to make an emergency landing, and a Belarus official later confirmed it was hit by a rocket-propelled grenade.
President Abdullahi Yusuf, head of Somalia's weak transitional government, is trying to secure Mogadishu before a reconciliation conference scheduled for April 16. Attacks and violence have plagued the city since the transitional government seized it from Islamic militants in December of last year.
The reconciliation conference would bring together members of some of the country's different factions to discuss how to end the violence and move forward as a country. However, the president has refused to include representatives of the popular Islamic Court Union.
