White House contender Mitt Romney says conflict in the Middle East is now more likely than when president Barack Obama took office.
Mr Romney used a Virginia military institute speech to deliver a scathing critique of Mr Obama's foreign policy record, which is sure to be on the agenda at next week's second presidential debate.
The Republican challenger says the United States, under Mr Obama's leadership, has been happy to follow rather than lead in the Middle East, increasing the threat of instability in the region.
He says Mr Obama's tenure has caused "great strains" with Israel, leading to a "dangerous situation" that has emboldened Iran.
Mr Romney says Iran has exploited the divisions as it pursues a nuclear program that has "never posed a greater danger" to America's friends, allies and the US itself.
"We can't support our friends and defeat our enemies in the Middle East when our words are not backed up by deeds, and the perception of our strategy is not one of partnership but of passivity," he said.
"It's clearer that the risk of conflict in the region is higher than when the president took office."
Mr Romney said that if elected he would help arm the Syrian opposition and tighten sanctions further on Iran to prevent it building a nuclear weapon.
"I'll work with our partners to identify and organise those members of the (Syrian) opposition who share our values and then ensure that they obtain the arms they need to defeat Assad's tanks and helicopters and fighter jets," he said.
"I'll put the leaders of Iran on notice that the United States and our friends and allies will prevent them from acquiring nuclear weapons capability.
"I will not hesitate to impose new sanctions on Iran and tighten the sanctions we currently have."
Mr Romney also said he would try to find a path to peace between Israel and the Palestinians.
"In this old conflict, as in every challenge we face in the Middle East, only a new president brings the chance to begin anew," he said.
"There's a longing for American leadership in the Middle East and it's not unique to that region, it's broadly felt by America's friends and allies in other parts of the world as well."
Mr Romney's campaign had flagged the speech as an important milestone, but observers were still looking for more specifics.
"In many ways it's still not entirely clear on some of the key issues whether there's that much of a difference in a policy sense between Barack Obama and Mitt Romney," Foreign Policy magazine editor-in-chief Susan Glasser said.
The race to the White House is tightening ahead of next month's election, with the latest Gallup poll putting both men at 47 per cent.