Obama made the comments Saturday as US Secretary of State John Kerry was in Paris to smooth over differences with France, which has pressed for greater guarantees that an agreement will stop Iran from gaining a nuclear weapon, and a bruising speech to Congress earlier in the week by Israel's Prime Minister Benjamin Netanyahu.
"If there is no deal then we walk away," Obama said in the interview, which aired on "CBS News Sunday Morning" and in expanded form on the network's "Face the Nation" show.
"If we cannot verify that they are not going to obtain a nuclear weapon, that there's a breakout period so that even if they cheated we would be able to have enough time to take action -- if we don't have that kind of deal, then we're not going to take it," he said.
Netanyahu, who charged in an impassioned speech to Congress Tuesday that a nuclear deal would pave the way for an Iranian bomb, showed no sign of budging in an interview on the same "Face the Nation."
- Netanyahu 'a lot more circumspect' -
"I do not trust inspections with totalitarian regimes," he said.
"And so I'd be a lot more circumspect. In fact, what I'm suggesting is that you contract Iran's nuclear program, so there's less to inspect."
Obama said the Iranians have negotiated seriously and progress has been made "in narrowing the gaps, but those gaps still exist.
"And I would say that over the next month or so, we're going to be able to determine whether or not their system is able to accept what would be an extraordinarily reasonable deal, if in fact, as they say, they are only interested in peaceful nuclear programs.
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"And if we have unprecedented transparency in that system, if we are able to verify that in fact they are not developing weapons systems, then there's a deal to be had, but that's going to require them to accept the kind of verification and constraints on their program that so far, at least, they have not been willing to say yes to."
Obama said the negotiations on Iran's nuclear program were gaining "greater urgency because we have been negotiating for over a year."
"And the good news is during this period Iran has abided by the terms of (an interim) agreement. We know what is happening on the ground in Iran. They have not advanced their nuclear program.
"So we're not losing anything through these talks. On the other hand, you get to a point in negotiations where it is not a matter of technical issues any more, it is a matter of political will."
- French concerns -
In Paris, Kerry also agreed with the French that there were still gaps to overcome in the "critical weeks" ahead.
"We want an agreement that's solid," Kerry told reporters after meeting with French Foreign Minister Laurent Fabius.
Fabius had expressed his concerns over the deal on Friday, saying "as regards the numbers, controls and the length of the agreement, the situation is still not sufficient."
Iran has long denied seeking to arm itself with an atomic bomb, insisting its nuclear program is for energy production and other civilian purposes.