Sheikh Sharif Sheikh Ahmed, the chief of the ARS, and Nur Hassan Hussein, the Somali interim prime minister, signed agreements at United Nations-sponsored talks in neighbouring Djibouti on June 9.
Talks included a three-month truce which should have come into force on July 9.
Ahmed said the alliance, which is based in Eritraea, wanted the withdrawal of Ethiopian forces from Somalia and a deployment of UN troops before implementing the Djibouti agreement.
Sheikh Hassan Dahir Aweys, an influential religious scholar and senior figure of the opposition Union of Islamic Courts, rejected the Djibouti deal, but is yet to comment on the latest endorsement.
Wracked by violence
Somalia had been wracked by violence since the 1991 when Mohamed Siad Barre, the former president, was removed from power.
The weak interim Somali government brought in Ethiopian forces as military support in late 2006 to fight against the Islamist movement that had control of much of the south and centre of the country.
These troops have been an unpopular presence and the subject of almost daily attacks. Aid workers have also been targeted in recent months.