The AU currently has about 3,200 soldiers from Uganda and Burundi deployed in Somalia, where two years of fighting have killed more than 16,000 civilians and driven a million from their homes.
Al-Shabab and other anti-government groups have regularly launched attacks on the troops, aiming to drive the peacekeepers out of the country.
Nicolas Bwakira, the AU's special representative for Somalia, said earlier this month that the forces needed to be reinforced fast.
The Nigerian military has long vowed to send a battalion of 850 officers and soldiers, but the deployment has been repeatedly delayed.
Nigeria's defence minister said on Friday that Nigeria is still assessing when to send the troops.
"When Nigeria committed herself to sending troops in 2006 or thereabout, it was for peacekeeping, because the situation then could be handled," Shettima Mustapha
told the Vanguard newspaper.
"But over the years, the situation has changed ... What we have are various groups controlling several areas. It means you will fight to enforce peace there. So if you go there, you will be fighting several groups," he said.
Meanwhile on Friday, Omar Abdirashid Sharmarke, who was appointed as prime minister last week, named members of his cabinet.
Sharmarke has appointed Abdulkadir Ali Omar, the deputy chairman of the Islamic Courts Union (ICU), as interior minister, according to a list from the prime minister's office.
Sharif Hassan Sheikh Aden, founder of the Alliance for the Re-Liberation of Somalia (ARS) and an ally of President Ahmed, was named deputy prime minister and finance minister.
Aden was a key player in the process that saw the ARS join parliament and elect Ahmed president.
Somalia has not had an effective central government since 1991, when Mohammed Siad Barre was forced from power.