President Barack Obama clashed with Republican presidential contenders on Tuesday over how to thwart Iran's nuclear ambitions, accusing them in the harshest terms yet of beating the drums of war while failing to consider the consequences.

The Republicans, competing to be the most hawkish on Iran and favourable to Israel before the powerful pro-Israel lobby group AIPAC, blasted Obama for a policy they said was too weak in backing the Jewish state and confronting Tehran.

The Iranian nuclear standoff dominated Obama's first news conference of the year, held on the same day as the Republicans' pivotal Super Tuesday round of presidential primaries and during a Washington visit by Israeli Prime Minister Benjamin Netanyahu.

Insisting there is time for sanctions to work against Tehran, Obama made clear that in talks with Netanyahu he cautioned against acting prematurely against Iran. Washington fears that Israel could attack Iran's nuclear sites in coming months.

There's no doubt that those who are suggesting or proposing or beating the drums of war should explain clearly to the American people what they think the costs and benefits would be, Obama said as he bluntly hit back at Republican critics.

He chided them for speaking with casualness and bluster on the prospects for war with Iran, which has emerged as one of the few salient foreign policy issues in a presidential campaign expected to hinge heavily on Obama's economic record.

But Republican presidential hopefuls issued strident calls for a more aggressive approach than the Democratic president's assertion that U.S. military action remains an option if sanctions and diplomacy fail to prevent Tehran from developing nuclear weapons.

If Iran doesn't get rid of nuclear facilities, we will tear them down ourselves, vowed Rick Santorum, a conservative former Pennsylvania senator.

Santorum, former Massachusetts governor Mitt Romney, former House Speaker Newt Gingrich and U.S. Representative Ron Paul were busy battling for U.S. votes on Tuesday, the biggest day so far in the race to be the Republican challenger to Obama in the November 6 election. Ten states were holding primary contests on what is called Super Tuesday.

Santorum, Romney and Gingrich also took time out to speak to the annual gathering in Washington of AIPAC, which includes some of Israel's staunchest U.S. supporters. Santorum appeared in person and the other two spoke by satellite link.

The political stakes are high in the increasingly bitter domestic debate on Iran. Both sides want the support of Jewish voters and other pro-Israel constituencies. But another war in the Middle East could spike global oil prices and hurt the fragile U.S. economic recovery.

OBAMA SAYS CHOICE NOT IMMINENT

Seeking to ally fears that war may be imminent, Obama said the notion that the United States needed to make a choice on military action in coming weeks or months was not borne out by facts. But he insisted the United States would not countenance a nuclear-armed Iran.

Netanyahu told Obama - who has backed Israel's right to self-defence - that he had not made a decision to attack Iran, but also gave no sign of backing away from military action.

Differences remained between the two leaders on how to confront Iran, with Netanyahu warning Obama that time was running out.

Romney told AIPAC that if he becomes U.S. president, Israel will know that America stands at its side in all conditions and in all consequences.

But Romney, who as the winner of the last five primary contests carried the momentum going into Tuesday's primary voting, was slightly more circumspect than Santorum in his choice of words about how he would respond as president to an Iranian nuclear weapons program.

Romney said he would bring Obama's procrastination on the matter to an end by imposing further crippling sanctions on Tehran, and I will make sure Iran knows of the very real peril that awaits if it becomes nuclear.

As president, I will be ready to engage in diplomacy. But I will be just as ready to engage our military might, Romney added.

Obama accused his Republican opponents of playing politics with the lives of U.S. military personnel.

What's said on the campaign trail, those folks don't have a lot of responsibilities. They're not Commander-in-Chief, he said. This is not a game. There's nothing casual about it.

Still, Defence Secretary Leon Panetta pressed a threat that Obama has also issued in more direct terms in recent days. Military action is the last alternative when all else fails, he told AIPAC. But make no mistake, when all else fails, we will act.

If Romney was somewhat cautious, Gingrich threw caution to the wind. Speaking by video link from his native state of Georgia, where he was campaigning, the former speaker said the U.S. State Department policy was wrong and the U.S. intelligence community was timid.

As far as Gingrich was concerned, Iran had already crossed a red line with its nuclear program, and was deepening their commitment to nuclear weapons while we talk.

And speaking in a week in which Obama has been appealing to Netanyahu to allow international sanctions to work against Iran, Gingrich indicated that if Israel wanted to attack Iran's nuclear sites, they did not need to check with him first.

I will provide all available intelligence to the Israeli government, ensure that they have the equipment necessary, to reassure them, that if an Israeli prime minister decides that he has to avoid the threat of a second Holocaust through pre-emptive measures, that I would require no advance notice, he said.

Israel's government is adamant it will not let Iran build an atomic bomb, and has been pushing Washington to pile pressure on Tehran to force it to abandon its nuclear program. The Jewish state is believed to be the only nuclear power in the region. Tehran, which denies seeking a nuclear bomb, has called for the destruction of the state of Israel.

The Republicans' speeches contrasted sharply with Obama's address to AIPAC on Sunday, in which he warned against loose talk of a war with Iran that he said was driving up oil prices.

Netanyahu sharpened his own tone toward Iran in remarks to AIPAC on Monday night, saying he had waited for diplomacy to work but we cannot afford to wait much longer.

(Additional reporting by Alister Bull, Caren Bohan and Missy Ryan; editing by Mohammad Zargham)