Accusing President Barack Obama of naivete on Iran, Republican presidential challenger Mitt Romney promised Thursday that if elected president he would "prepare for war" with the Islamic republic.

In a commentary published in the Wall Street Journal, Romney said he would back up US diplomacy "with a very real and very credible military option," deploying carrier battle groups to the Gulf and boosting military aid to Israel.

"These actions will send an unequivocal signal to Iran that the United States, acting in concert with allies, will never permit Iran to obtain nuclear weapons," he wrote.

Romney, a frontrunner in the race for the Republican presidential nomination, keyed his column to a International Atomic Energy Agency report this week citing "credible evidence" that Iran had worked on a nuclear explosive device.

Iran denies it is developing nuclear weapons and insists its nuclear program is for generating electricity, but the report has prompted calls in the West for tougher UN sanctions and demands by Israel for world to act to prevent Tehran for getting nuclear weapons.

Romney said the United States "needs a very different policy."

"'Si vis pacem, para bellum.' That is a Latin phrase, but the ayatollahs will have no trouble understanding its meaning from a Romney administration: If you want peace, prepare for war," he said.

He stopped short of advocating military action against Iran, but attacked the Obama administration's diplomatic and sanctions-oriented approach to Tehran's nuclear program as "a case study in botched diplomacy."

"Whether this approach was rooted in naivete or in realistic expectations, can be debated. I believe it was the former," Romney wrote.

He criticized the administration for failing to get Moscow's support for tougher action against Tehran as the price for a "reset" in US-Russian relations, and Obama's refusal to meddle during Iran's Green Revolution of 2009.

"A proper American policy might or might not have altered the outcome; we will never know," he wrote. "But thanks to this shameful abdication of moral authority, any hope of toppling a vicious regime was lost, perhaps for generations."

With the US military tied down in Iraq and Afghanistan, the Obama administration has played down a US military option against Iran, opting instead for diplomacy and sanctions.

Robert Gates, Obama's Republican defense secretary until earlier this year, warned repeatedly against the use of military force, arguing it would only drive the Iranian program deeper underground.

"The reality is there is no military option that does anything more than buy time," Gates told CNN in 2009.