Speaking at the closure, new AU chairman John Kufuor said several nations had pledged troops - but only 4,000 out of a required 8,000.
The force is due to replace withdrawing Ethiopian soldiers, whose intervention swept Islamists from power last month.
Meanwhile, Somalia's leader has agreed to host a reconciliation conference.
At the start of the meeting in Addis Adaba, 4,000 troops were committed by Uganda, Nigeria, Ghana and Malawi.
We appeal to member states to contribute and we are still expecting them to answer
AU chairman John Kufuor
Mr Kufuor, who is president of Ghana, said there had been no increase in that number, but he hoped other countries would come forward.
"We appeal to member states to contribute and we are still expecting them to answer," he said, adding that the deployment would "commence as soon as possible".
Mr Kufuor was elected as the new AU chairman on Monday, bypassing Sudan's President Omar al-Bashir because of the conflict in Darfur.
Mr Bashir has repeatedly denied backing the Janjaweed militias, accused of carrying out widespread atrocities in Darfur and says the problems there have been exaggerated.
Anarchy fears
Earlier on Tuesday, Somalia's President Abdullahi Yusuf announced a reconciliation conference - the date still to be confirmed - saying it would give all Somalis "a fair chance" to participate in efforts to return to lasting stability.
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
Benin: Considering
Burundi: Considering
Tanzania: Considering
Rwanda: Considering
South Africa: Not sending troops
He said the conference would include clan and religious leaders but he did not say whether moderates from the ousted Union of Islamic Courts (UIC) would be invited.
European Union Development Commissioner Louis Michel said this meant the EU would be able to release 15m euros ($20m) to fund the peacekeeping force.
The EU, the US and the UN have all urged Mr Yusuf to include moderate Islamists in his administration. The US has offered to provide air support for the peacekeeping force.
In December, thousands of Ethiopian soldiers were sent to help the weak Somali interim government oust the UIC, which controlled much of southern and central Somalia for six months.
But Ethiopia says it is seeking an early withdrawal from the country and has already begun pulling some of its troops out.
The fear, says the BBC's Adam Mynott, is that unless insecurity is contained quickly, Somalia will slip back to the anarchic misrule which has prevailed in the country for the past 16 years.
Meanwhile, a previously unknown Somali group threatened to fight any peacekeepers.
An Islamic website posted a message from the "Popular Resistance Movement" that read: "Somalia is not a place where you can come to earn a salary - it is a place where you can die."