"I absolutely agree that given the progress that has been made over the last several weeks that Gaddafi and his regime need to understand that there will not be a let-up in the pressure that we are applying," he said in a joint news conference with British Prime Minister David Cameron.
"I believe that we have built enough momentum that as long as we sustain the course that we are on that he is ultimately going to step down," he added.
"And we will continue to work on our progress to achieve that, so we have not put forward any artifical timeline in terms of how long this will take place."
Both Obama and Cameron reiterated calls for Gaddafi to go.
Cameron said: "I believe we should be turning up that pressure (on Libyan government) and on Britain's part we will be looking at all the options of turning up that pressure."