At the White House, spokesman Jay Carney used slightly stronger language than he had in the last day, telling reporters at his daily briefing that the United States has no doubt that Assad was behind the chemical weapons attack last week in Syria.
"I'm not aware of any doubt that exists," he said. "And we believe that a careful review of the facts leads to the conclusion that the regime was behind this."
Carney said the United States will release an intelligence reporter later this week that will prove that Assad was behind the attack.
Carney said Obama is still considering what the U.S. response will be. Those options, he said, include use of force.
"What the president is engaged in is a process of deciding, as he consults with international allies and as his administration consults with Congress, about what the appropriate response to this flagrant violation of international norms should be," he said.
"There must be a response. We cannot allow this kind of violation of an international norm, with all the attendant grave consequences that it represents, to go unanswered. What form that response will take is what the president is assessing now with his team."
Carney declined to say whether Obama will seek permission for his actions from Congress or the United Nations because he said that would be speculation.
"I'm not going to, you know, make legal justifications for actions that haven't been decided upon. When the president makes a decision about what the appropriate response for the United States is, we will and he will provide ample context for the decision that he makes."
But he said the response was in response to attack, and not regime change.
"The options that we are considering are not about regime change. They are about responding to a clear violation of an international standard that prohibits the use of chemical weapons."