Burundi and Uganda want air and marine units to support their troops
African Union officials in Somalia have told the BBC pro-government forces in the capital Mogadishu are plugging gaps left by departing Ethiopian troops.
A BBC correspondent says pro-government forces face an array of insurgents which has so far proved stronger.
African Union peacekeepers on Sunday said they may have no option but to leave unless their mandate is boosted and their troop numbers bolstered.
Ethiopia's army began pulling its 3,000-strong force out last week.
The withdrawal came two years after Ethiopia helped the transitional government oust Islamists from Mogadishu.
BBC world affairs correspondent Mark Doyle says the move raises the prospect of a possible power vacuum in the capital, where there has not been a functioning central government since 1991.
A spokesman for the AU peacekeeping force in Mogadishu, Major Ba-Hoku Barigye, told the BBC a mixture of forces from the government and the Alliance for the Reliberation of Somalia (ARS) were taking over some positions from the exiting Ethiopian army.
The ARS is made up partly of groups which used to fight the government, but which have now come to a deal with it.
Major Barigye said AU peacekeepers would not be directly taking up positions previously held by the Ethiopians because this could draw them into confrontation with the anti-government insurgents.
Earlier, another AU official had told the BBC their peacekeepers would take over some positions left by the Ethiopian military.
Our correspondent says the armed insurgents are a mixture of Islamists, nationalists and traditional Somali clans.
The situation in Mogadishu is now on a knife-edge, he adds.
The Islamist and nationalist insurgents have vowed to overthrow what remains of the government, whose president resigned last month.
Meanwhile, the UN said more than 40 civilians were killed and some 50,000 displaced by fighting between rival Islamist militias in central Somalia last week.
There are rumours that the Ethiopians have been arming clan militias to take on the hardline Islamist group al-Shabab.
The insurgency has revived in a more extreme form
The militias have reportedly managed to defeat the radicals during several clashes in central Somalia and on the border with Ethiopia.
After a meeting on Sunday in Burundi's capital, Bujumbura, the Ugandan and Burundian defence ministers said the AU's force in Somalia (Amisom) needed more troops, as well as air force and marine units and a stronger mandate.
They gave the AU two weeks to meet their conditions.
Burundi Defence Minister Gen Germain Niyoyankana said more troops were needed "very quickly" and added:
"If our conditions are not met, it's normal that our troops will be withdrawn", reported AFP news agency.
Ghana, Nigeria and South Africa have not yet met their pledges to send forces.
The 3,600-strong Amisom force has faced frequent attack from Islamist insurgents in Mogadishu.
Ethiopia's departure follows an agreement with the Somali transitional government and the more moderate wing of the Islamist-led political opposition during UN-sponsored reconciliation talks in Djibouti.
Government forces only control parts of Mogadishu and the town of Baidoa.
Large parts of the Somali capital are already flattened by artillery fire and 2008 saw roughly half the civilian population flee fighting.