The transition in Central African Republic, overrun by rebels in March, must not exceed 18 months, the African Union warned Friday.
"The transition in CAR is not meant to last more than 18 months" until free and fair elections are held, the AU's peace and security chief Ramtane Lamamra said.
That would mean the mineral-rich nation would have to hold polls by September 2014 at the latest.
A rebel coalition known as Seleka seized power on March 24, forcing President Francois Bozize to flee.
The coalition, which has since been accused of going on a looting and killing spree, is made up of four rebel groups which have been operating in the northern part of the country since 2003.
While regional leaders set up a peace plan that was accepted by Seleka, CAR's new strongman Michel Djotodia, who named himself president after the coup, has gone against the accords, observers say.
Lamamra said a force of 400 troops from several countries in the central African region currently deployed in CAR, the Mission for the Consolidation of Peace in the Central African Republic (MICOPAX), will be turned into a larger African force.
"As of August 1 the decision is to transform MICOPAX into an African-led mission to support CAR," Lamamra said.
The AU's Peace and Security Council has authorised the force to have 3,500 armed personnel.
Two thousand troops from central African nations have so far been committed or are already serving, Lamamra said. The AU is also looking for a further 1,500 men from nations in other parts of the continent.
A former French colony, Central African Republic has been chronically unstable for the greater part of the past two decades.