REPUBLICAN White House hopeful Mitt Romney has branded Iran's nuclear drive as President Barack Obama's "biggest failure" and promised a very different approach if he wins the US election.

In an interview broadcast on Sunday, Romney said: "Perhaps the biggest failure is as it relates to the greatest threat that America faces and the world faces, which is a nuclear Iran," Romney said in the interview with NBC's Meet the Press.

"The president has not drawn us further away from a nuclear Iran and, in fact, Iran is closer to having a weapon, closer to having nuclear capability than when he took office," Romney said.

"President Obama had a policy of engagement with (President Mahmoud) Ahmadinejad. That policy has not worked, and we're closer to a nuclear weapon as a result of that.

"I will have a very different approach with regards to Iran. And it's an approach which, by the way, the president's finally getting closer to. It begins with crippling sanctions. That should have been put in place long ago."

The US and Europe have imposed strict sanctions while talks between Tehran and major world powers are in deadlock.

Obama has vowed Iran will not get a nuclear weapon, and his White House says it will be able to tell if the Islamic Republic makes significant progress towards the critical steps needed to build one.

Iran insists its nuclear program is exclusively peaceful and denies it is trying to build a nuclear weapon.

A recent report by the International Atomic Energy Agency said Iran had installed more than 1000 new uranium enrichment centrifuges in a bomb-proof nuclear bunker in Fordo, near the holy city of Qom.

Originally published as Iran is Obama's 'biggest failure': Romney