Ban said a key UN Security Council declaration condemning Syria for the crackdown was "the clear message of the international community" to Assad, who has been refusing to take the UN secretary general's telephone calls for several weeks.
"The world has watched the deteriorating situation in Syria with the most profound concern. But the events of the past few days have been brutally shocking," Ban told reporters after the Security Council adopted the statement.
"Just continuing like this is not sustainable. He cannot and they cannot carry on like this killing their people."
The UN secretary general made a new call for Assad to halt all violence, carry out promised reforms and let in international humanitarian teams and human rights investigators.
The Security Council statement called for Ban to report on events in Syria within seven days. Ban said he would mobilize all UN agencies and work with non-government groups to gather information on the crackdown.