Hillary Clinton, the front-runner for the 2008 Democratic presidential nomination, backed the Bush administration's sanctions against Iran but stressed that she opposes US military action against the Islamic republic.
Clinton's comments late on Tuesday during a debate among seven Democratic hopefuls underscored how President George W Bush's hardening stance on Iran is shifting the US foreign policy debate, just over a year before the election to replace him in the White House.
Several rivals attacked Clinton, the former first lady and current US senator from New York, for supporting a Senate resolution in September that some Democrats say could be read as authorising Bush to launch war on Iran.
In imposing sanctions on Iran, the US administration last week declared an elite military unit a supporter of terrorism and targeted Iranian banks in an effort to curb Tehran's nuclear activities.
"I am not in favour of this rush for war, but I'm also not in favour of doing nothing," Clinton said during the televised debate at a university in Philadelphia. "I prefer vigorous diplomacy and I happen to think that economic sanctions are part of vigorous diplomacy."
But, she said, "We've got to do everything we can to prevent George Bush and the Republicans from doing something on their own to take offensive military action against Iran."
Bush's October 17 warning that Iran's obtaining the know-how to make a nuclear weapon could lead to World War III has prompted Democratic charges that the administration is sabre-rattling like in the build-up to war in Iraq. The White House rejects that notion.
"There is no intention of bombing Iran. We are on a diplomatic track," Bush's spokeswoman Dana Perino said on Tuesday.
Clinton's approach reflected her effort to take a presidential, centrist stance against her more left-leaning rivals, a balancing act crucial to her bid to become the first female US president.
Democrats and Bush's Republicans have held a series of debates this year, but the stakes are getting higher as both parties gear up for the first presidential preference vote in the mid-western farm state of Iowa on Jan 3.
Clinton faced sniping from her closest rivals, Senator Barack Obama from Illinois state and former vice presidential candidate John Edwards. They portrayed her as evasive and lacking political principles and attacked her early support for the war in Iraq in the Senate.
Asked by NBC television newscaster Brian Williams whether there was a "red line" for military action against Iran, Clinton replied: "I am not going to speculate about when or if they get nuclear weapons."
But she also insisted, "I will do everything I can to prevent Iran from developing a nuclear bomb."
Clinton has opened a wide lead in recent weeks over second-place Obama, whose youthful charisma and calls for change have struck a chord especially among young Democrats.
He hinted before the debate that he would get tougher on Clinton to spark his campaign, but it was Edwards who had the sharpest barbs.
He accused Clinton of defending "a broken system that's corrupt" in Washington, saying "one thing one time and something different at a different time" and of cozying up to Bush on Iran.
"Remember the prelude to Iraq?" Edwards said. "We have to stand up to this president."