Mortar shells hit the airport in Somalia's capital, Mogadishu, Tuesday, shortly after two cargo planes delivered the first group of African Union peacekeepers.

Witnesses say unknown gunmen fired at least seven mortars, causing people to scatter at the airport. It was not immediately clear if there were any casualties.

The attack occurred a few hours after the arrival of about 350 troops from Uganda. That country has pledged 1,500 troops in all to the AU peacekeeping force in Somalia.

The force will replace Ethiopian soldiers who helped the Somali government drive a rival Islamist movement out of Mogadishu and other strongholds in December.

Ethiopian troops fought a gunbattle Tuesday with unidentified assailants near a Somali government base in Mogadishu. Witnesses say at least one civilian was killed.

Mogadishu has witnesses almost daily attacks in recent weeks from insurgents believed to be linked to the Islamist movement.

In two separate attacks late Monday, gunmen in Mogadishu killed at least five people, including a top police official and a Muslim cleric.

Islamist leaders have vowed to continue attacking the government, the Ethiopians and any peacekeepers who set foot in Somalia.

The African Union had hoped to gather 8,000 troops for the peacekeeping force, but member nations have so far pledged only half that number.

Some information for this report was provided by AFP, AP and Reuters.