The signing of the agreement on Tuesday marked the end of a bad-tempered fifth session of the Abuja conference, which will now break up, having made little more progress than previous attempts to resolve the 29-month-old conflict.
"This is just a beginning," chief AU mediator Salim Salim said at the talks' final public session, adding: "Some formidable challenges lie ahead."
But he insisted: "I believe that by adopting this declaration the message from Abuja will be towards the ending of the conflict in Darfur and the realisation of peace, stability and security for all."
More than 180,000 civilians have been killed and 2.5 million driven from their homes during fighting in the western Sudanese region, creating what UN agencies have dubbed the "world's worst humanitarian disaster".
The AU has sent a peacekeeping force to monitor the oft-broken ceasefire signed by the parties in April last year, but five rounds of peace talks in the Nigerian capital have failed to find a political solution.
The government and two fractious rebel groups - the Justice and Equality Movement and the Sudanese Liberation Army/Movement - will now reconvene for further dialogue at a later date, AU officials said.