At the same time he must signal support for Israel, but not so much support that the Israelis see the buildup as an opportunity to strike the Iranian nuclear facilities, which Obama’s team believes could set off a war without significantly setting back the Iranian program.

A key motivation for “Olympic Games,” the covert effort to undermine Iran’s enrichment capability with cyber attacks, has been to demonstrate to the Israelis that there are more effective ways to slow the program than to strike from the air.

This delicate signaling to both Iran and Israel is a complex dance. Sen. John Kerry, D-Mass., chairman of the Foreign Relations Committee, said that the administration must strike a fine balance between positioning forces to deter Iran, but not inadvertently indicate to Iran or Israel that an attack on Iran’s nuclear sites is imminent or inevitable.

“There are a lot of expectations to manage,” Kerry said in an interview. “People need to know you’re serious, but you must also leave room for peaceful resolution.”

There is little evidence that the increased pressure is having the desired effect. Negotiations with Iran are at a stalemate, although a group of Iranian, U.S. and European experts are expected to meet in Istanbul on Tuesday to review a recent U.S. proposal and Iranian response.

Iran has strenuously resisted efforts to force it to give up enrichment of uranium, starting with production of a type that is considered relatively close to bomb grade.

Responding to the tightening of Western sanctions, Iran announced Monday it would consider proposed legislation to disrupt traffic in the Strait of Hormuz as well as missile tests, in a drill clearly intended as a warning to Israel and the United States.

The legislation calls for Iran’s military to block any oil tanker en route to countries no longer buying Iranian crude because of the embargo. It was unclear whether the legislation would pass or how Iran would enforce it.

Senior Pentagon and military officials acknowledge that Iran has the capability to close the strait, at least temporarily, and the additional mine-clearing forces can be viewed as both concrete and spoken evidence of Washington’s commitment to make sure any closing is as brief as possible.

Defense Department officials stressed that the recent reshaping of U.S. forces in the Persian Gulf region should not be viewed as solely about the potential nuclear threat from Iran.

“This is not only about Iranian nuclear ambitions, but about Iran’s regional hegemonic ambitions,” the senior Defense Department official said. “This is a complex array of American military power that is tangible proof to all of our allies and partners and friends that even as the U.S. pivots toward Asia, we remain vigilant across the Middle East.”

While U.S. ground troops have been withdrawn from Iraq, a force equivalent to an extra Army combat brigade has remained in Kuwait, officials said. It could have many roles to contain regional instability, but Iran is a primary concern.

While it always is difficult to read Iran’s intentions, senior U.S. Navy officers have noted that Iranian ships in the Persian Gulf have refrained recently from provocative behavior.

“Things have been, relatively speaking, quiet,” said Adm. Jonathan W. Greenert, the chief of naval operations, assessing actions by Iranian Navy vessels over “the last couple of months.”

But that was without the pressure of the new sanctions. Already, Iran is exporting far less oil every day than a year ago: about 1.5 million barrels a day compared to 2.5 million before the gradual imposition of earlier sanctions.

While Iranian vessels have avoided any confrontations with allied warships in recent weeks, Iran expects to equip its ships in the Strait of Hormuz soon with shorter-range missiles, a Revolutionary Guards commander said Friday, according to the semiofficial Mehr news agency.