He said countries around the world share "grave doubts" about Iran's presentation of its nuclear intentions.
The president said such solidarity on the part of a wide range of nations is "not typical" and speaks to the situation that Iran now finds itself in.
"Anybody who's been following responses to Iran would have been doubtful just a few months ago that that kind of rapid response was possible," he said, noting that China and Russia have called for an investigation.
On Friday morning, Mr. Obama and the leaders of Britain and France held a press conference to discuss the existence of a previously-unknown secret Iranian nuclear facility and call for it to be opened.
He said Iran "is breaking rules that all nations must follow, endangering the world non-nuclear proliferation regime ... and the security of the world."
Iranian President Mahmoud Ahmadinejad responded that his country had complied with United Nations rules and said the president's charges were a "mistake."
"I'm sure they'll definitely feel sorry about it. I think they probably already regret it and will be regretting it more down the road," he said."
The president said at his press conference Friday that Iran is "on notice" and that "they are going to have to come clean and they are going to have to make a choice" at meetings with the United States on October 1st.
"Are they willing to go down the path which I think ultimately will lead to greater prosperity and security for Iran, giving up the acquisition of nuclear weapons, and deciding that they are willing to abide by international rules and standards in their pursuit of peaceful nuclear energy," he said, "or will they continue down a path that is going to lead to confrontation?"
Asked about a military response to Iran's nuclear program, the president said he doesn't rule anything out. But he said "my preferred course of action is to resolve this in a diplomatic fashion."
In what appeared to be a reference to Russia, the president noted that some nations have indicated that they are now open to tougher sanctions against Iran, something they were "reluctant to even discuss" months ago. He said nations around the world recognize that Iran's behavior is "an important enough issue to peace and stability in the world" and that they must act together.
"Iran would make a mistake in ignoring the call for them to respond in a forthright and clear manor," the president said, adding that the country's actions over the next several months will determine if they are further isolated from the world or can "rejoin the international community."
Responding to a question from CBS News' Chip Reid, the president declined to offer details about possible sanctions. But he said that Iran's "actions contradict its words" and said the united front presented by the world means the country will have to "pay attention."
"The international community, I think, has spoken," he said. "It is now up to Iran to respond."