MOGADISHU, Somalia — Mortars fired by Islamic militants slammed into Somalia’s airport as the president was boarding a plane yesterday, sparking battles that killed at least 24 people when return fire hit residential areas and a market, officials said.

A militant leader vowed to avenge the civilian deaths and threatened retaliatory attacks in two African countries that supply troops to the African Union peacekeeping mission stationed in Mogadishu.

President Sheik Sharif Sheik Ahmed was unhurt, and his plane took off safely, police said.

But the civilian deaths are fueling a growing anger toward African Union peacekeeping forces in Mogadishu to help protect the U.N.-backed government.

Somalia’s capital sees near-daily bloodshed, as a powerful insurgent group with links to al-Qaida tries to overthrow the fragile government and push out some 5,000 AU peacekeepers. Both sides have been accused of indiscriminate shelling.

“Soldiers from Uganda and Burundi soldiers are our enemy. They often massacre our people. We will not let them go unpunished, but will target them in Kampala and Bujumbura,” the capitals of the two countries, said Sheikh Ali Mohamed Hussein, a leader of al-Shabab, the militant group linked to al-Qaida that controls much of southern Somalia.

The shelling started soon after insurgents fired at Mr. Ahmed’s plane, said police spokesman Abdullahi Hassan Barise.

Yesterday’s violence — deadlier than many recent clashes in this once-beautiful seaside city — follow a pattern that witnesses say is becoming all too common. First, insurgents fire at government or AU targets. Then those forces respond by shelling insurgent bases, most of which lie in residential areas. The result is that most of those killed in Somalia’s war are civilians.