At least two people have been killed, including a Somali police officer, during an hour-long gunbattle in the capital, Mogadishu.
Gunmen have staged several attacks on Somalia's police force
The clash began when a group of gunmen threw a grenade at the police, witnesses say.
This is the latest in a series of violent attacks in the city, since Islamist forces were driven out of the city last month.
The African Union is putting together a peacekeeping force for Somalia.
The clashes late on Sunday followed an attack on two police stations earlier in the day.
No-one has claimed responsibility for the attacks but the government has blamed the remnants of the Union of Islamic Courts (UIC). It says some 3,500 Islamist fighters remain in Mogadishu.
The AU force would replace the Ethiopian troops, who helped oust the UIC and install the government in Mogadishu.
Nine battalions proposed - up to 9,000 troops:
Uganda: 1,500 troops offered, subject to parliamentary approval
Malawi: Up to 1,000 troops offered
Nigeria: 1,000 troops offered
Ghana: Reportedly offered troops
South Africa:Will not send troops
The Ethiopians have started to withdraw their troops, with about a third due to leave in the coming days.
Some analysts fear an increase in violence if the Ethiopians leave before the AU peacekeepers arrive.
So far, only Uganda, Malawi and Nigeria have pledged to send troops to Somalia.
But the US would lend air support to the proposed 8,000-strong force, US Assistant Secretary of State for African Affairs Jendayi Frazer told the AFP news agency.
"We are ready to provide airlift and contracting airplanes for the African peacekeeping force in Somalia," she said.
A court in Somalia's neighbour, Kenya, is due to decide whether to deport a one of the main UIC financial backers to Somalia.
Abubakar Omar Aden, a 72-year old businessman, has pleaded guilty to illegally entering Kenya but says his life would be in danger if he was sent home.
Kenya has deported 57 people to Somalia in recent days, in a move criticised by the International Committee of the Red Cross.
About 200 Ethiopian soldiers left Mogadishu last week
Spokesman Yazdi Pedram said there was no legal basis for repatriating them after they fled to Kenya during the recent fighting.
Government spokesman Abdirahman Dinari said they would be brought to court but said that most of them investigated so far were foreign passport-holders.
He said they include Arabs, Ethiopians, Eritreans and holders of US and European passports.
He also said the wives of key international terror suspects and three children had been sent to Somalia.
"The wife of Mohamed Fazul, one of the guys wanted for the 1998 terror bombings on US embassies in Kenya and Tanzania and the wife of Yassin Omar, one of the suspects of the [failed 21 July 2005] London bombings, are among the suspects we hold," he said.
The UIC denied repeated accusations that they were harbouring al-Qaeda operatives.