Barack Obama has yet to take a decision on whether to go ahead with a military strike against Syria, the White House has said.
Ahead of a Commons debate in Westminster, the United States government said the first step towards a military response in Syria would be the public release of its intelligence regarding the use of chemical weapons by President Bashar Assad.
State Department deputy spokeswoman Marie Harf said there was no doubt about who was responsible for last week's chemical weapons attack.
She told BBC Newsnight: "We are making our own decisions, on our own timeline. Right now we have determined that the Assad regime did in fact use chemical weapons against his own people; our intelligence assessment will be shared with the public this week at some point. But let there be no question about who is responsible here and the fact that President Obama will be making decisions about our own response, on our own timeline."
Ms Harf insisted Mr Obama had not yet made up his mind about whether to proceed with military action. "What I'm saying is President Obama and the National Security team are right now looking at the range of options on how to respond. No decision has yet been made on how we will," she said.
"But I think what has been clear in the past few days is that it has been too long, the Assad regime has continued shelling the area of the attack, for the UN investigation at this point to be credible. We saw today that it was too dangerous for the investigation team to even go and investigate. So we believe that it's time now that we will be making a decision about the appropriate response very soon."
Asked if Mr Obama would wait until Westminster had convened and had the opportunity to discuss the proposals, Ms Harf said the US was working to its own timetable but was in close contact with its British allies.
She said: "Firstly, the president hasn't made a decision about what our response will be. We, the president and the secretary of state, have been in close contact with our counterparts in London about this exact issue. I don't have a timeline for you about when any potential response might happen at this point."
Meanwhile, the former British ambassador to Washington Sir Christopher Meyer said a military strike against Syria would be "the decision from hell".
Speaking in a pre-recorded interview on Newsnight he said: "It cannot be in the British national interest to see Assad disintegrate under the pressure of cruise missile attacks, and whatever else may be done, such that his stocks of chemical weapons fall out of his control into the hands of the extremist Jihadists among the rebels. This is why this decision on what to do next is truly the decision from hell."