A series of public statements and private communications from the Israeli leadership in recent weeks set off renewed concerns in the Obama administration that Israel might be preparing a unilateral military strike on Iran, perhaps as early as this fall.

But after a flurry of high-level visits, including one by Defense Secretary Leon E. Panetta to Israel on Wednesday, a number of administration officials say they remain hopeful that Israel has no imminent plans to attack and may be willing to let the United States take the lead in any future military strike, which they say would not occur until next year at the earliest.

The conversations are part of delicate negotiations between the United States and Israel that have intensified over the past month. On Wednesday they continued with Mr. Panetta, who appeared with the Israeli defense minister, Ehud Barak, and declared that the United States would stand by Israel if Iran developed a nuclear weapon.

“We have options that we are prepared to implement to ensure that that does not happen,” Mr. Panetta said. Standing with Mr. Barak in front of an Israeli antirocket missile battery in the southern town of Ashkelon, about five miles from the Gaza border, Mr. Panetta made clear what he meant. “My responsibility is to provide the president with a full range of options, including military options, should diplomacy fail,” he said.

In the last three weeks, a steady stream of administration officials have flown to Israel to meet with Prime Minister Benjamin Netanyahu, among them Secretary of State Hillary Rodham Clinton and Thomas E. Donilon, the national security adviser. The trips were in part planned for other reasons — Mrs. Clinton and Mr. Panetta were going to Egypt to meet with the new president and so diplomatically could not ignore Israel — but administration officials say that there has been an intense effort to stay in close contact with Israel and abreast of its intentions.

The visits, deliberately or not, also sandwiched in Mitt Romney, the presumptive Republican presidential nominee, who was in Jerusalem two days before Mr. Panetta. Mr. Romney, who received a short briefing from the American ambassador in Israel but had no other substantive communication with the administration, appeared to take a harder line against Iran than President Obama has.

In Jerusalem, Mr. Netanyahu continued on Wednesday with his tough rhetoric of recent days, arguing that sanctions against Iran were largely useless. “Right now the Iranian regime believes that the international community does not have the will to stop its nuclear program,” Mr. Netanyahu said. “This must change and it must change quickly because time to resolve this issue peacefully is running out.”

Administration officials say that Israeli officials are less confrontational in private and that Mr. Netanyahu understands the consequences of military action for Israel, the United States and the region. They say they know he has to maintain the credibility of his threat to keep up pressure on the United States to continue with sanctions and the development of military plans.

“The more the Israelis threaten, the more we respond by showing them that we will take care of the problem if it comes to that,” said Martin Indyk, a former American ambassador to Israel and director of foreign policy at the Brookings Institution.

Mr. Panetta met separately on Wednesday with Mr. Netanyahu, Mr. Barak and Shimon Peres, the Israeli president. Administration officials say the Americans and Israelis shared the latest intelligence on Iran, coordinated implementation of the most recent sanctions and discussed military options. Mr. Panetta said on Tuesday in Cairo that he was not taking any American attack plans to show to the Israelis.

He also said that any American strike would be a last resort. “We have to exhaust every option, every effort, before we resort to military action,” Mr. Panetta said at the Ashkelon missile battery, which is part of the Iron Dome defense system in part paid for by the United States.

On Wednesday, Mr. Panetta used some of his sharpest language on Iran, as if to assure the Israelis that the Obama administration could be equally tough.

“This is not about containment,” Mr. Panetta told reporters at the start of his meeting with Mr. Peres. “This is about making very clear that they are never going to be able to get an atomic weapon.”

In Israel, there remains feverish speculation that Mr. Netanyahu will act in September or early October. Besides the prime minister’s fear that Israel’s window of opportunity will close soon, analysts cite several reasons for the potential timing: Israel does not like to fight wars in winter. Mr. Netanyahu feels that he will have less leverage if President Obama is re-elected, and that if Mr. Romney were to win, the new president would be unlikely to want to take on a big military action early in his term.

“If I were an Iranian, I would be very fearful of the next 12 weeks,” said Efraim Halevy, a former chief of Israel’s intelligence agency and national security adviser.

Others made light of the constant visits from the United States. “The visitors are actually baby sitters to make sure the unpredictable kids do not misbehave,” said Efraim Inbar, the director of the Begin-Sadat Institute for Strategic Studies.

American defense officials and experts in Israel say that because Israel does not have a bomb powerful enough to penetrate Iran’s underground uranium-enrichment facilities, an independent strike would be likely to set the nuclear program back only one or two years, at most. That has led to major dissent among Israel’s security professionals over the wisdom of such an attack. The Pentagon, in contrast, has the munitions, bombers, missiles, stealth aircraft and drones that would cause far more extensive damage.

David Makovsky of the Washington Institute for Near East Policy, who recently spent time in Israel talking to government officials, said that Israel’s longstanding doctrine of self-reliance makes American promises to act later if all else fails less effective. Instead, he said, Israel needs to be convinced that if it waits, it can still retain the option to act independently.

“Make Israel not believe that it’s two minutes to midnight,” Mr. Makovsky said. “If Israel is so convinced that its window of action is shutting, then maybe you try to enlarge Israel’s window. You say, ‘Here, we know there are some things you need. But we don’t want you to use them until several months ahead.’ ”

The Obama administration is eager to prevent an Israeli attack partly to avoid a major foreign policy crisis during the American presidential campaign and partly because officials say an Israeli strike could set off a new conflagration in the region. If Iran retaliated by launching missiles at Tel Aviv that killed thousands of Israelis, administration officials say the United States would be under enormous pressure to defend Israel and respond, and would then be pulled into another war in the Middle East.

Mr. Netanyahu and other Israeli officials have argued that Iran makes progress in enriching nuclear fuel every day, enhancing its capability to withstand a strike and keep any nuclear weapons program on track. Iran denies the intent to develop nuclear weapons and says its program is for peaceful purposes.

The Israeli news media have been filled in recent days with speculation about a strike. One article said that the Obama administration had vowed to strike within 18 months, another reported continuing concerns in the security establishment here about the effectiveness of an Israeli strike, and a third said that Mr. Donilon, Mr. Obama’s national security adviser, had revealed details of the American attack plans during his visit. The articles did not make clear where those accounts came from, but they contributed to a growing atmosphere of expectation.

“Everybody’s leaking like crazy right now — that doesn’t mean there will be a strike, but it means we’re closer to a decision,” said Amos Harel, defense correspondent for the left-leaning Israeli newspaper Haaretz, who estimated the chance of an attack before November at 50 percent. “It’s probably a more crucial junction than it was ever before.”