President Barack Obama has rejected criticsm by his Republican rivals over his policy on stopping Iran's nuclear programme.—Reuters photo

“I think it's very important to remember, particularly given some of the political noise out there, that this administration has systematically imposed the toughest sanctions on Iran ever,” Obama told reporters.

“When we came into office the world was divided. Iran was unified and moving aggressively on its own agenda,” he said.

“Today Iran is isolated and the world is unified and applying the toughest sanctions that Iran's ever experienced, and (that) is having an impact inside of Iran.”

The president, assailed in recent weeks by Republican foes who portrayed Obama as unable or unwilling to toughen US policy on Iran, added that he was still “considering all options” with respect to ways to get Tehran to rein in its controversial nuclear program.

“Iran understands that they have a choice,” Obama said.

“They can brave that isolation by acting responsibly and foreswearing the development of nuclear weapons which would still allow them to pursue peaceful nuclear power... or they can continue to operate in a fashion that isolates them from the entire world,” he said.

“And if they are pursuing nuclear weapons, then I have said very clearly that is contrary to the national security interests of the United States, it is contrary to the national security interests of our allies, including Israel, and we are going to work with the world community to prevent that.”

On Wednesday Obama faced a swarm of protest on Middle East and Iran policy from Republicans eager to challenge him in the November 2012 elections.

Several of the Republican White House candidates accused Obama of coddling Tehran.

“Ultimately, regime change is what's going to be necessary” in Iran, former Massachusetts governor Mitt Romney said, while his rival Newt Gingrich bluntly called for “regime replacement” and vowed to “fund every dissident group in the country.”