U.S. Secretary of State Hillary Clinton says Syria cannot be peaceful, stable or democratic with President Bashar al-Assad in power.

Speaking to reporters Thursday in Istanbul, Clinton said Assad has “doubled down on his brutality and duplicity” and that the time has come for the international community to plan for a post-Assad Syria.

“We have to unite the international community behind a plan that is achievable and keeps faith with those inside Syria who are protesting and demonstrating, suffering and dying for their universal human rights,” Clinton said.

UN briefing

Her comments come as United Nations-Arab League mediator Kofi Annan is due to brief the U.N. Security Council and General Assembly on the situation in Syria, as activists have accused pro-government militia and security forces of carrying out a new massacre in the central province of Hama.

The activists said at least 78 people, including women and children, were killed on Wednesday.  They said some of those killed in the villages of al-Kubeir and Maazarif were stabbed to death and at least 12 bodies were burned.

The Britain-based Syrian Observatory for Human Rights said ‘shabiha’ militiamen armed with guns and knives carried out the attack after regular troops had shelled the area.

The exiled opposition Syrian National Council called on the Free Syrian Army and other armed opposition groups to respond by escalating their resistance.

British Prime Minister David Cameron called the reported attack “brutal and sickening,” and said if true, then the international community must do more to condemn “absolutely” the government of President Assad.

The Syrian government blamed a terrorist group for the violence.