Obama stole some of the political limelight from Republican presidential hopefuls, holding his first White House news conference in five months as voters went to the polls in nationwide Super Tuesday nominating contests.
A day after Israeli Prime Minister Benjamin Netanyahu said he could not wait "much longer" for diplomacy on Iran to work, Obama also slammed Republicans for "big talk" and "bluster" and failing to consider the costs of war.
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"This notion that somehow we have a choice to make in the next week or two weeks or month or two months is not borne out by the facts," Obama said, arguing a "window" for diplomacy could forestall an Iranian bomb.
Obama has also warned however that he will not tolerate Iran obtaining a nuclear weapon, would not attempt a containment policy and is prepared to strike at the appropriate time.
He spoke after world powers responded to Iran's new willingness to discuss the nuclear issue with an offer of talks, which Obama said would "quickly" show whether the Islamic republic was serious about avoiding war.
Obama, seeking a second term in November, argued that Iran was now feeling the "bite" of tightening sanctions though cautioned he did not expect a breakthrough in a first set of negotiations.
He also slammed Republican candidates for their hawkish statements demanding military action on Iran, after leading Republican candidate Mitt Romney earlier said "thugs and tyrants" only understood American readiness to use power.
"This is not a game, and there's nothing casual about it," Obama said.
"When I see the casualness with which some of these folks talk about war, I'm reminded of the costs involved in war," he said, talking of visiting wounded soldiers and writing condolence letters to families of fallen warriors.
"Sometimes we bear that cost, but we think it through. We don't play politics with it," he said, noting he and not his Republican foes bore the heavy responsibilities of US commander-in-chief.
"When we haven't thought it through, and it gets wrapped up in politics, we make mistakes."
Obama described the crackdown in Syria which has killed thousands of civilians as "heartbreaking" and "outrageous" and predicted that President Bashar al-Assad would eventually fall, like other regional tyrants.
But he warned that unlike Libya, where the United States and allies prevented a massacre, there was no case for outside intervention.
"For us to take military action, unilaterally, as some have suggested, or to think that somehow there's some simple solution, I think is a mistake," Obama said, pointing out that the Libyan operation attracted a UN Security Council mandate and cooperation from regional governments.
"This is a much more complicated situation."
Obama noted that he was holding his press conference on Super Tuesday, and beamed when asked what he would like to say to Romney, replying, "Good luck tonight," keeping his powder dry for a likely future clash later.
Romney, the Republican favorite, hopes to use key contests in Ohio and Tennessee which were among 10 states voting on Tuesday to position himself as presumptive nominee for November's presidential election.
In Massachusetts, where he planned to await results, Romney said Obama had made "some extraordinary errors" with respect to Iran.
Obama did wade into controversy over conservative talk show icon Rush Limbaugh -- a hate figure for some -- who called a student a "slut" for arguing her health plan at a Catholic-affiliated college should provide birth control.
"I don't know what's in Rush Limbaugh's heart, so I'm not going to comment on the sincerity of his apology," Obama said.
"All decent folks can agree that the remarks that were made don't have any place in the public discourse."
Recent surveys have shown Obama's personal job approval rating at around 50 percent, a crucial historical threshold for presidents seeking reelection, after three White House years marred by economic gloom.
