"He wavers from seeming to believe that mediation and meetings without preconditions can solve the world's intractable problems, to advocating rash, unilateral military action without co-operation from our allies in the most sensitive region of the world," Clinton said in Washington DC.
Clinton has lost the last 11 Democratic presidential nominating contests to Obama and many analysts have said that she needs to win the forthcoming Ohio and Texas polls on March 4 to retain a chance of winning the nomination.
After a debate with Obama last Thursday in Texas in which she said she was honoured to share the stage with him, Clinton has toughened her message in the past few days.
She criticised Obama on Monday, saying that the US needed a leader with more foreign policy experience.
Americans took a chance on George Bush, the current president, and have been disappointed and should not take a chance again, she said.
"We've seen the tragic result of having a president who had neither the experience nor the wisdom to manage our foreign policy and safeguard our national security. We can't let that happen again," she said.
A Quinnipiac University poll published on Monday showed Clinton leading Obama in Ohio by 51 per cent to 40 per cent among probable Democratic voters.
Two weeks ago, Clinton had led by 55 per cent to 34 per cent, a sign that the momentum Obama has gained from 11 straight victories was paying dividends there.
Obama's campaign accused the Clinton camp of "shameful, offensive fear-mongering" after the picture of Obama dressed in a traditional Somali robe and turban appeared on the Drudge Report website.
The picture was taken during a visit by the candidate to his father's homeland of Kenya in 2006.
David Plouffe, Obama's campaign manager, said the photo represented "exactly the kind of divisive politics that turns away Americans of all parties and diminishes respect for America in the world".
The website said the photo had been circulated by Clinton aides, a claim denied by her campaign, which said the Obama team should be "ashamed" for suggesting the image could prove divisive in the hard-fought election.