U.S. Secretary of State Hillary Clinton is discussing the Syria crisis Friday with United Nations-Arab League envoy Kofi Annan, who has warned of an all out civil war in the country if the situation does not change course.

The meeting in Washington comes one day after the U.N. said its unarmed monitors were shot at and blocked from investigating the site of a recently reported massacre in central Hama province. Activists said at least 78 people, including women and children, were killed this week in the village of Mazraat al-Qubeir.

Annan, whose peace plan brokered in April has not been implemented, warned Thursday against allowing “mass killings to become part of everyday reality in Syria.”

He called on the divided U.N. Security Council and the rest of the international community to unite and act immediately to intensify pressure, especially on Syrian President Bashar al-Assad's government.

During a visit to Istanbul Thursday, Secretary Clinton said the Syrian leader has “doubled down on his brutality and duplicity,” and that the time has come for a post-Assad Syria.

Opposition activists have accused pro-government forces in Syria of carrying out Wednesday's massacre in Hama, which if confirmed, would be the fourth such mass slaying of civilians in the last two weeks. Syria's government has blamed a terrorist group for the violence.

The White House Thursday accused the Syrian government of orchestrating “the outrageous targeted killings of civilians, including women and children…as reported by multiple credible sources.”

Thousands of Syrians have died in attacks and clashes since the cease-fire was put in place, and the presence of hundreds of U.N. observers has not been able to stem the violence.