Barack Obama does not anticipate Iranian nuclear deal falling through

President Barack Obama, facing a tough opposition from Republicans and from some of his fellow Democrats on the landmark Iranian deal, on Sunday said he does not anticipate the agreement failing in the Congress.

"I have a general policy on big issues like this: not to anticipate failure. And I am not going to anticipate failure now because, I think, we have the better argument," he said.

"I just go back again and again to those who are opposed to the deal, can't just say we want a better deal. They can't just say we're going to be tougher. This is serious," Obama told CNN's Fareed Zakaria.

"It requires us asking tough questions and engaging in a substantive conversation about how are we to achieve what even my fiercest critics would acknowledge should be a shared goal, which is preventing Iran from getting a nuclear weapon," Obama said.

Britain, France, Germany, Russia, China and the US signed an agreement last month with Iran to ensure Tehran does not acquire a nuclear bomb, in return for relief from sanctions that were wearing down its economy.

Obama said if Congress were to reject the Iranian nuclear deal deal, then that central goal would be harder to achieve.

"And the international unity that we've brought about over the last several years would fray, not just with respect to sanctions, but with respect to the world's attitude about US leadership and how they gauge who's at fault in this dispute between the US and Iran," he said.

Obama said the issue is does the rest of the world take seriously the US' ability to craft international agendas, to reach international agreements, to deliver on them in ways that garner respect and adherence from other countries.

"That's continually tested. And what Congress needs to understand is, is that we are the most powerful country on Earth. But our power does not simply come from the fact that we've got the biggest military.

"Our power derives from the fact that since World War II, we have put together international institutions that have served our interests but have also served the interests of the world," Obama said.

"And as much as people may complain about the US, they still recognise that we have been able to operate on the basis of principles and values and build human institutions that function effectively and fairly around the world."

He said: "if we stop doing that, then our power will be diminished, no matter how big our military budget is. And it will become a much more dangerous world. That's why I don't intend to lose on this."