Israeli Prime Minister Benjamin Netanyahu, left, and U.S. Defense Secretary Leon Panetta shake hands during a meeting at the Prime Minister's office in Jerusalem, Aug. 1, 2012. (Sebastian Scheiner/AP)

Defense Secretary Leon E. Panetta stressed Wednesday that if economic sanctions do not compel Iran to end its nuclear program, the United States will have to consider military options to destroy it.

Panetta’s repeated emphasis on pursuing other options if diplomacy fails did not mark a change in policy but gave his remarks a harder edge than his previous statements.

His comments came amid deepening concern that Israel could launch a unilateral strike on Iran, and as Congress passed a new sanctions bill on Wednesday aimed at banks, insurance companies and shippers that assist Iran in selling its oil. There have been a series of visits to Israel by senior Obama administration officials, who are pressing the Israelis to give economic sanctions more time to persuade the Iranians to give up their nuclear ambitions.

Panetta described the recently imposed economic sanctions as “the toughest Iran has ever faced” and insisted that they are working. “The most effective way to stop Iran from obtaining a nuclear weapon is for the international community to be united, proving to Iran that it will only make itself less secure if it continues to try to pursue a nuclear weapon,” he said.

The defense secretary’s statements also came as presumptive Republican presidential nominee Mitt Romney is making the Obama administration’s policy toward Iran a campaign issue. While visiting Israel this week, Romney used sharp language, saying that “any and all measures” should be considered to prevent Iran from developing nuclear weapons.

Panetta appeared with Israeli Defense Minister Ehud Barak at a jointly funded U.S.-Israeli anti-rocket battery in southern Israel and then met with Prime Minister Benjamin Netanyahu in Jerusalem. But even as Panetta emphasized the Obama administration’s deep opposition to Iran’s nuclear program and the United States’ close partnership with Israel, the differences in the American and Israeli views on the need for urgent military action were clear.

Barak told reporters that the likelihood of sanctions curbing Iranian nuclear ambitions is “very, extremely low” and suggested that the Iranians are stalling for time as they move quickly to enrich the uranium they would need for a nuclear weapon.

“We have clearly something to lose by this stretch of time on which sanctions and diplomacy take place because the Iranians are moving forward,” he said, standing next to Panetta.

Netanyahu reiterated that message after his meeting with Panetta. “However forceful our statements, they have not convinced Iran that we are serious about stopping them,” Netanyahu said. “Right now, the Iranian regime believes that the international community does not have the will to stop its nuclear program. This must change quickly, because the time to resolve this issue peacefully is running out.”

In remarks that appeared designed to increase pressure on the Iranians and reassure the Israelis, Panetta said repeatedly that the United States has developed military options to thwart the Iranian nuclear program if sanctions fail.

“We will not allow Iran to develop a nuclear weapon. Period,” Panetta said after his meeting with Netanyahu. “And we will exert all options in the effort to ensure that that does not happen.” In his appearance with the Israeli defense minister, Panetta said that it is his responsibility to “provide the president with a full range of options, including military options, should diplomacy fail.”

Unlike the U.S. military, the Israel Defense Forces do not have tankers capable of refueling warplanes in flight, nor is Israel’s arsenal of bunker-busting bombs thought to be as effective as that of the United States at taking out deeply buried targets. Those shortcomings could limit the effectiveness of any unilateral action by the Israelis against the Iranian nuclear program.

Panetta spent the morning touring the anti-rocket battery, developed by the Israelis with U.S. assistance and more than $200 million in U.S. aid. Last week, President Obama pledged an additional $70 million to help Israel bolster the Iron Dome system, which is designed to shoot down short-range rockets from Gaza and Lebanon. Panetta called the system a “game changer” for the Israelis and said it had shot down more than 80 percent of the rockets fired in recent months at Israeli cities.The anti-rocket system would not be effective against longer-range Iranian missiles, which can be countered only with more sophisticated theater missile-defense systems.

A unilateral Israeli strike on the Iranian program would be likely to trigger large reprisal strikes by Iran against Israel and U.S. targets in the Middle East. There would be intense pressure on the Obama administration to provide for Israel’s defense.

Panetta’s quick tour of the Iron Dome system was designed to highlight the close partnership between Israel and the United States.

“This is the strongest alliance that we have . . . and we will continue to strengthen the military relationship,” Panetta said.

While touring the Iron Dome with Panetta, Barak was asked whether Israeli and the U.S. clocks were “ticking at different speeds” when it came to Iran.

Barak responded:“That’s correct, this is a well-known fact. . . . We all understand the same intelligence, we all use the same language, and still it’s true that there are certain differences. America, even when it thinks differently than us, understands that the state of Israel and the government of Israel are those who ultimately have to make the decisions in matters that are vital to the security of the country.”

Joel Greenberg in Jerusalem contributed to this report.