Asked whether she believed Assad was "evil," State Department spokeswoman Victoria Nuland said that while she "personally would consider what he has done evil" it was not a label the US government was putting on him.
"He is beyond brutal. It is inhumane what he has done to his own people," she told journalists.
"We've certainly said we think he`s lost the legitimacy to lead, that he is one of the most brutal actors on the world stage today."
On Sunday, in a defiant speech to a packed theater Assad called for peace in Syria on his terms outlining, in his first speech in seven months, his vision for a way out of the 21-month conflict that has shattered his country.
The United Nations says more than 60,000 people have been killed since rebels launched their uprising against Assad`s rule.
"I don't think anybody who... is guilty of the kinds of crimes against your own people that he`s guilty of could be considered rational by any human sense of the word," Nuland said.
Assad`s plan for an end to violence, dialogue with opposition elements he deemed acceptable, and a vow to stand fast against those he called "terrorists" and their foreign backers drew wild applause from his Damascus audience.
But it offered little realistic prospect of ending what has become a civil war, and Nuland said the delirious scenes just showed "as we've seen all over the world, renting a mob is not that difficult."