US President Barack Obama hit back at 2012 election rival Mitt Romney on Sunday over Iran, suggesting that people that try to simplify the nuclear issue are playing politics.

Romney, a former Massachusetts governor and frontrunner for the Republican presidential nomination, intimated at a foreign policy debate on Saturday that he could stop Iran from getting nuclear weapons while Obama could not.

"Is this an easy issue? No," the president responded at a press conference in Hawaii after hosting a summit of Asia-Pacific leaders. "Anybody that claims it is is either politicking or doesn't know what they're talking about."

"Not only the world but the Iranian regime understands very clearly how determined we are to prevent not only a nuclear Iran, but also a nuclear arms race in the region," he said.

Obama said the world was united in opposing Iran's nuclear program and that Washington was working with China and Russia to further tighten the screw on the regime in Tehran.

Tension between Iran and its two principal foes, Israel and the United States, has risen since the release of a UN report saying there was "credible" evidence suggesting Iran's atomic program was being used to research putting nuclear warheads in ballistic missiles.

Obama discussed Iran with Russian President Dmitry Medvedev and Chinese President Hu Jintao at bilateral meetings over the weekend in Hawaii.

"What I did was to speak with President Medvedev as well as President Hu and the three of us entirely agree on the objective, which is making sure that Iran does not weaponize nuclear power and that we don't trigger a nuclear arms race in the region," Obama said.

"In terms of how we move forward, we will be consulting with them carefully over the next several weeks."

The United States and European Union have called for further sanctions but Russia and China have been reluctant.

"The sanctions have enormous bite and enormous scope and we're building off a platform that has already been established," Obama said.

"The question is are there additional measures that we can take, and we are going to explore every avenue to see if we can solve this issue diplomatically."

Israeli media in recent weeks have been full of chatter suggesting a pre-emptive strike on Iran's nuclear facilities was being seriously considered by Prime Minister Benjamin Netanyahu and Defence Minister Ehud Barak.

"We are not taking any options off the table because it is my firm belief that an Iran with nuclear weapons would pose a security threat not only to the region but also to the United States," Obama said.

Republican opponents have sharply criticized Obama on Iran, particularly over his decision early in his term to offer talks to the Islamic regime.

Obama said Sunday that his "strong preference" was still to resolve the issue diplomatically. Tehran denies it is trying to build nuclear weapons and insists its work is purely for civilian energy purposes.