WASHINGTON -- The United States has quietly moved significant military reinforcements into the Persian Gulf to deter the Iranian military from any possible attempt to shut the Strait of Hormuz and to increase the number of fighter jets capable of striking deep into Iran if the standoff over its nuclear program escalates.

The deployments are part of a long-planned effort to bolster the U.S. military presence in the Gulf region, in part to reassure Israel that in dealing with Iran, as one senior administration official put it last week, “When the president says there are other options on the table beyond negotiations, he means it.”

But at a moment that the United States and its allies are beginning to enforce a much broader embargo on Iran’s oil exports, meant to force the country to take seriously the negotiations over sharply limiting its nuclear program, the buildup carries significant risks, including that Iran’s powerful Islamic Revolutionary Guards Corps could decide to lash out against the increased presence.

The most visible elements of this buildup are Navy ships designed to vastly enhance the ability to patrol the Strait of Hormuz — and to reopen the narrow waterway should Iran mine it to prevent Saudi Arabia and other oil exporters from sending their tankers through the vital passage.

The Navy has doubled the number of minesweepers assigned to the region, to eight vessels, in what military officers describe as a purely defensive move.

“The message to Iran is, ‘Don’t even think about it,’ ” one senior Defense Department official said. “Don’t even think about closing the strait. We’ll clear the mines. Don’t even think about sending your fast boats out to harass our vessels or commercial shipping. We’ll put them on the bottom of the gulf.”

Like others interviewed, the official spoke on the condition of anonymity because of the delicacy of the diplomatic and military situation.

Since late spring, stealthy F-22 and older F-15C warplanes have moved into two bases in the Persian Gulf to bolster the combat jets already in the region and the carrier strike groups that are on constant tours of the area. The additional attack aircraft give the U.S. military greater capability against coastal missile batteries that could threaten shipping, as well as the reach to strike other targets deeper inside Iran.

And the Navy, after a crash development program, has moved a converted amphibious transport and docking ship, the Ponce, into the Persian Gulf to serve as the Pentagon’s first floating staging base for military operations or humanitarian assistance.

The initial assignment for the Ponce, Pentagon officials say, is to serve as a logistics and operations hub for mine-clearing. But with a medical suite and helicopter deck, and bunks for combat troops, the Ponce eventually could be used as a base for Special Operations forces to conduct a range of missions, including reconnaissance and counterterrorism, all from international waters.

For President Barack Obama, the combination of negotiations, new sanctions aimed at Iran’s oil revenues and increased military pressure is the latest test of what the White House calls a “two-track” policy against Iran. In the midst of a presidential election campaign in which his opponent, Mitt Romney, has accused him of being weak on the Iranian nuclear issue, Obama seeks to project toughness without tipping into a crisis in the region.