The Security Council unanimously approved a resolution Saturday expanding the number of U.N. cease-fire observers in Syria from 30 to 300 and demanding an immediate halt to the violence that has been escalating since the government and opposition agreed to end hostilities more than a week ago.

The resolution is the first authorizing unarmed U.N. military observers to go into a conflict area, and it gives Secretary-General Ban Ki-moon authority to decide when to deploy the additional monitors, based on developments on the ground including “the consolidation of the cease-fire.’’

Ban accused Syrian President Bashar Assad on Thursday of failing to honor the cease-fire, expressing dismay that increased violence is claiming more lives. In a statement Saturday from his spokesman welcoming adoption of the resolution, the U.N. chief again demanded that Syria end all violence and “the gross violations of the fundamental rights of the Syrian people,’’ and send its troops and heavy weapons back to their barracks.

The resolution establishes a United Nations Supervision Mission in Syria, to be known as UNSMIS, “comprising an initial deployment of up to 300 unarmed military observers as well as an appropriate civilian component’’ for an initial period of 90 days to monitor the cessation of violence and the implementation of international envoy Kofi Annan’s six-point peace plan.

The resolution merged rival Russian and European texts and dropped a European threat of non-military sanctions against Syria if it fails to withdraw troops and heavy weapons from towns and cities. Instead, it uses language from the resolution adopted last Saturday authorizing deployment of the 30-strong advance team of observers which expresses the council’s intention to assess implementation of the new resolution “and to consider further steps.’’