Publicly, administration officials sidestepped the question.

“We have a number of ways to communicate our views to the Iranian government, and we have used those mechanisms regularly on a range of issues over the years,” said Tommy Vietor, a spokesman for the White House’s National Security Council. “I’m not going to get into the details of those communications or mechanisms.”

Defense Secretary Leon E. Panetta said the United States had not increased its forces in the Persian Gulf because there was enough American military strength there already. “We obviously always continue to make preparations to be prepared for any contingency, but we are not making any special steps at this point in order to deal with the situation,” Mr. Panetta said. “Why? Because, frankly, we are fully prepared to deal with that situation now.”

Israel, which regards Iran as its most dangerous enemy, has repeatedly dropped hints recently that it reserved the right to attack Iranian military targets if it concluded that Iran had the capacity to construct a nuclear weapon. But Mr. Barak told Israel’s Army Radio on Wednesday that “we don’t have a decision to go forward with these things.”

“We don’t have a decision or a date for taking such a decision,” he said. Iran has repeatedly denied that its nuclear fuel enrichment activities are for military use, saying they are meant only for civilian energy and medical purposes. It has called the Israeli threats of attack part of a broader Western-led policy of intimidation, and has accused Israel in particular of orchestrating a clandestine subterfuge campaign, including the assassination of its nuclear scientists. The deputy director of Iran’s main uranium processing operation was assassinated in Tehran last Wednesday, the fifth Iranian scientist with connections to the nuclear program to be killed since 2007.

Mr. Barak’s remarks came ahead of an imminent visit to Israel by the American chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey. Israeli news media commentators have suggested that General Dempsey was coming in part to warn Israel against going it alone in striking Iran’s nuclear facilities. Mr. Barak denied that suggestion. He also reiterated the Israeli assessment that Iran had not started building nuclear weapons.