Obama asked members of Congress including Democrats not to pursue new sanctions while talks are underway.
"I will veto a bill that comes to my desk," Obama warned.
"The chances that we can actually get a diplomatic deal are probably less than 50-50. Iran is a regime that is deeply suspicious of the West, deeply suspicious of us. In the past, they have surreptitiously and secretly advanced aspects of this programme. We have huge differences with them on a whole range of issues," Obama told reporters at a joint White House news conference with British Prime Minister David Cameron.
Responding to a question, Obama said under the interim deal that brought Iran to the table, they are were not supposed to initiate new sanctions.
"Now, you'll hear arguments, 'Well, these technically aren't sanctions. They're simply laws putting in place the possibility of additional sanctions.' I assure you, that is not how Iran would interpret it or our partners would interpret it," he said.
"So the likelihood of the entire negotiations collapsing is very high. And if that happens, there is no constraint on Iran, at that point, going back and doing exactly what it had been doing before they came to the table: developing a heavy water reactor that, once built, is extraordinary difficult to dismantle and very difficult to hit militarily, going back at underground facilities that are very hard to reach militarily, accelerating advanced centrifuges that shorten the time span in which they can achieve breakout capacity," Obama said.
He said if the Iranians say no and there is no deal, then, by all means, will sit down and work out what extra sanctions to put in place.
"Because we are absolutely united in a simple thought, which is a deal that takes Iran away from a nuclear weapon is better than either Iran having a nuclear weapon or military action to prevent it," he said.
Obama said the interim deal with Iran has frozen progress on their nuclear programme, rolled back, in some cases, the stockpiles of material that they had already accumulated, and
provided an insight into their programme that was unprecedented.
"We have people on the ground who are able to verify and inspect and tell us what exactly is going on. That's not just our assessment, that's the assessment of intelligence services
around the world, including the Israelis.
"So the agreement has held and the negotiations have been serious. We have not lost ground. Iran has not accelerated its programme during the time these negotiations have taken place.
In fact, Iran's programme has not only been in abeyance, but we've actually made gains in rolling back some of the stockpiles that they had," he said.


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