The main airport serving Somalia's capital has come under mortar fire, with three mortars landing on the airport, including one on the runway.
Violence has again been spiralling out of control in Mogadishu
Ethiopian troops are based there, but there are no reports of any injuries.
Dozens have been killed by insurgents since Ethiopia-backed government forces defeated the Islamists two months ago.
The Nigerian army has confirmed that it will start sending soldiers to Somalia as part of a peacekeeping force in mid-April - as Ethiopian forces leave.
Interim Somali President Abdullahi Yusuf has told the BBC he expects the African Union (AU) force to be in his country in the next few weeks.
Dozens of people living in the area around the airport were reported to be trying to escape the fire-fight there.
According to an eyewitness, more than 20 unknown gunmen started firing mortars into the airport compound.
The interim government says some 3,500 Islamists are left in Mogadishu, whom it accuses of leading insurgent attacks.
Some Islamists vowed to start a guerrilla war after they were driven out of the capital in December.
But after 16 years without an effective government, Mogadishu is awash with weapons.
Also in Mogadishu, police are investigating the killing of two officials in separate incidents on Wednesday.
Mogadishu Mayor Hussein Adde Gabow said it was not clear if the two deaths were related.
"District Commissioner of Yaqshiid in North Mogadishu Muhyadin Hassan was shot dead and minutes later the deputy district commissioner of Wadajir, Abdi Omar Gogooye, was shot in the south-west of the city," Mayor Gabow told the AFP news agency.
AU suicide threat
The AU is finalising plans to deploy the first battalion of peacekeepers into Mogadishu after the United Nations Security Council sanctioned the move this week.
President Yusuf said they would help with training, and to guard vital installations, but he told the BBC that Somali forces, and not AU troops, would be expected to deal with "troublemakers".
He also said "apart from some trainers", all Ethiopians would move back across the border as soon as the AU force arrived.
Some 4,000 soldiers from Uganda, Nigeria, Malawi, Ghana and Burundi are ready to replace Ethiopian troops who have started to withdraw.
But the Nigerian government are the first contributing nation to give a deployment date.
The army said 850 troops would be deployed to Somalia by the middle of April in three phases.
An insurgent group calling itself the Popular Resistance Movement in the Land of Two Migrations has threatened any AU troops, in a warning posted on a website that was associated with the ousted Islamists.
"We promise we shall welcome them with bullets from heavy guns, exploding cars, and young men eager to carry out suicide attacks against these colonial forces," the message said.