AU Peace and Security Commissioner Said Djinnit made the announcement after a meeting with United Nations and European Union officials.
The meeting took place a day before Somali President Abdullahi Yusuf Ahmed was due to hold talks with his ally, Ethiopian Prime Minister Meles Zenawi.
The seven-nation east African grouping, the Inter-Governmental Authority on Development, is planning to despatch a peacekeeping force to Somalia.
But it has run into problems ranging from an existing UN arms embargo on Somalia to opposition from the Islamic Courts militia, which has taken control of the capital, Mogadishu, and several other key areas.
"We have agreed to send an assessment mission that will assess all that is required for the peace-supporting mission and check the situation on the ground," Mr Djinnit said.
"But meanwhile the planning (for the peacekeeping force) should start," he added.
The AU's envoy to Somalia, Mohamed Ali Foum, said the team could go as early as next week.
Priority should be given to dialogue with the transitional government - which is largely powerless but has backed the peacekeeping force - "and all the parties in Somalia", Mr Djinnit said.
Opening for peace
He said the current situation, following the Islamic Courts militia's defeat of a US-backed alliance of warlords in southern Somalia after four months of clashes, was "a window of opportunity to reactivate the dialogue and the peace process in Somalia".
In New York, the UN envoy for Somalia called for urgent Security Council action to prevent Ethiopian military intervention in lawless Somalia.
The UN's special representative for Somalia, Francois Lonseny Fall, said the world body would this week send a security team to Jowhar, 90 kilometres (55 miles) north of Mogadishu, to assess security.
The visit would pave the way for another UN team that will discuss the humanitarian needs of the Somali population with the Islamist leaders.
The clashes have claimed at least 360 lives and wounded more than 2,000.
The European Commission's representative in Ethiopia, Tim Clarke, said, "We have reached a consensus to send an assessment team to Somalia very soon led by the AU and IGAD," which groups Djibouti, Eritrea, Ethiopia, Kenya, Sudan, Uganda and Somalia's interim government.
Hardline Islamic courts leaders have denied claims by the United States that they have links with extremist groups such as Al-Qaeda and have vowed to restore stability in Somalia.
The courts are slowly making headway in restoring order.
In addition, the Islamists imposed Sharia law in Jowhar, making good on their vows to bring Islamic theocracy to the shattered nation, home to 10 million people.
"I hope the new administration will quickly implement the Sharia law," courts chief Sheikh Shariff Sheikh Ahmed said hours after his militiamen raided cinemas, switched off generators and threw out audiences watching the soccer World Cup and western movies.
Islamic clerics also urged world powers to pressure Ethiopia to withdraw troops from Somali territory.
Troops cross border
They said several hundred Ethiopian troops had crossed into Somalia over the weekend and were moving toward the headquarters of the fledgling Somali transitional government in Baidoa, about 250 kilometres (155 miles) from the capital.
"We call on the international community to pressure Ethiopia remove its troops from Somalia territory in order to avoid another conflict," said Sheikh Ali Hassan, a Courts official.
Addis Ababa has denied deployment, but said it had reinforced its borders after unspecified provocations.
Together with the US-backed warlords they defeated, Islamic courts oppose the deployment of any peacekeeping force, fearing it will give an opportunity to neighbouring countries, notably historic foe Ethiopia as well as Kenya and Djibouti, to meddle in Somalia's affairs.
Since strongman Mohamed Said Barre was ousted in 1991, the country has been wracked by factional fighting, scuppering 14 internationally-backed efforts to restore a functional administration.
Earlier this month the AU renewed a call for the UN Security Council to lift an oft-violated 14-year-old arms embargo on Somalia to allow regional peacekeepers to be deployed.
The call was first made and rejected by the UN last year, but AU officials say the removal of the embargo is critical to the ongoing peace efforts.
"It is not realistic to go to Somalia without being armed", Mr Foum said.
Sudan's ambassador to Ethiopia Abduzeid El Hassan said, "As soon as the embargo on the arms will be lifted we are ready to send a battalion to Somalia. Uganda will be sending another battalion."
But Kampala and Khartoum have said they would deploy their troops if they are not welcomed by Somalis.
An attempt by UN and US peacekeepers to restore peace in Somalia between 1993 and 1994 ended in failure with the death of 18 rangers and several UN peacekeepers.