WASHINGTON - Israel has not yet decided whether to strike Iran over its suspect nuclear program, US Defense Secretary Leon Panetta said Tuesday, as Tehran dismissed the threat of an imminent attack.

Israeli Prime Minister Benjamin Netanyahu and Defense Minister Ehud Barak in recent days had stiffened their rhetoric on Iran, suggesting they were thinking more seriously of military action against Tehran's nuclear facilities.

When asked about the issue at a press conference, Panetta offered reassurances, echoing the White House line that there is still time to negotiate a solution to the nuclear showdown.

"I've said this before, I'll say it now -- I don't believe they made a decision as to whether or not they will -- they will go in and attack Iran at this time," Panetta told reporters.

"Obviously, they're an independent -- they're a sovereign country. They'll ultimately make decisions based on what they think is in their national security interest," he noted.

"The reality is that we still think there is room to continue to negotiate."

Earlier this month, Panetta visited Israel, where his pleas for more time to let diplomacy run its course received a less than warm welcome from Netanyahu and Barak.

Speculation in the Israeli press about a possible strike has since multiplied.

Israel insists that Iran is on the point of developing nuclear weapons, and says it reserves the right to act to prevent that. Tehran insists the program is solely for peaceful, civilian purposes.

Repeated rounds of talks between Iran and the permanent five members of the United Nations Security Council -- Britain, China, France, Russia and the United States -- plus Germany have failed to end the standoff.

Tehran has refused to make major concessions on its right to enrich uranium, a process used to make nuclear fuel but also the core of an atomic bomb.

US President Barack Obama has stressed that his position is that he will stop Iran from "acquiring nuclear weapons" a step further down the path to developing an atomic arsenal than the Israeli red line.

There is concern in Washington that a unilateral Israeli strike may not destroy Iran's underground nuclear facilities, could spark Iranian retaliation worldwide and may drag the United States into another war in the Middle East.

"Any kind of military action ought to be the last alternative, not the first," Panetta said Tuesday.

General Martin Dempsey, the chairman of the US Joint Chiefs of Staff, emphasized that any military strike would likely only delay Iran's nuclear program, not ruin it altogether.

"I may not know about all of their capabilities. But I think that it's a fair characterization to say that they could delay but not destroy Iran's nuclear capabilities," Dempsey told reporters.

On Tuesday, Iranian ministry spokesman Ramin Mehmanparast told reporters that Tehran was not taking the Israeli threats seriously.

"Even if some officials in the illegitimate regime (Israel) want to carry out such a stupid action, there are those inside (the Israeli government) who won't allow it because they know they would suffer very severe consequences from such an act," Mehmanparast said.