RUTHFULLY YOURS

By ADAM ENTOUS and JULIAN E. BARNES

The Pentagon is beefing up U.S. sea- and land-based defenses in the Persian Gulf to counter any attempt by Iran to close the Strait of Hormuz.

The U.S. military has notified Congress of plans to preposition new mine-detection and clearing equipment and expand surveillance capabilities in and around the strait, according to defense officials briefed on the requests, including one submitted earlier this month.

The military also wants to quickly modify weapons systems on ships so they could be used against Iranian fast-attack boats, as well as shore-launched cruise missiles, the defense officials said.

The readiness push is spearheaded by the military’s Central Command, which oversees U.S. forces in the Gulf region, these officials said. It shows the extent to which war planners are taking tangible steps to prepare for a possible conflict with Iran, even as top White House and defense leaders try to tamp down talk of war and emphasize other options.

The changes put a spotlight on what officials have singled out as potential U.S. shortcomings in the event of conflict with Iran. The head of Central Command, Marine Gen. James Mattis, asked for the equipment upgrades after reviews by war planners last spring and fall exposed “gaps” in U.S. defense capabilities and military preparedness should Tehran close the Strait of Hormuz, officials said.

The Central Command reviews, in particular, have fueled concerns about the U.S. military’s ability to respond swiftly should Iran mine the strait, through which nearly 20% of the world’s traded oil passes.

“When the enemy shows more signs of capability, we ask what we can do to checkmate it,” a U.S. military officer said. “They ought to know we take steps to make sure we are ready.”

Tensions with Iran have soared as the U.S. and its allies have tightened sanctions against the country over its nuclear program. Tehran has responded by threatening to close the strait. Israel has accused Iran of being behind a recent series of botched bombing plots targeting Israeli diplomats, a charge Iran denies. Iranian officials, in turn, accuse Israel and the U.S. of conducting a secret campaign to assassinate scientists working on Iran’s nuclear program. The U.S. has denied the accusation, while Israel has declined to comment

New suspicions over Iran’s nuclear ambitions emerged Friday. In a report, the International Atomic Energy Agency, the United Nations’ nuclear watchdog, said Iran has increased its stockpile of uranium that is enriched beyond the purity level needed for civilian power reactors, and begun producing it under a mountain of rock and soil that some U.S. and Israeli officials say could be immune from attack.

Iran denies it is trying to build atomic weapons. It refused this week to allow U.N. inspectors access to suspected weapons sites, adding doubts to prospects for negotiations.

The U.S. is concerned that Israel—which believes that Tehran will soon be able to assemble a weapon, and that time is running short to stop the bid—may choose to strike Iran by this autumn to stymie such a program. That, defense officials worry, could provoke retaliation that could prompt U.S. military action to defend its troops and key allies, and to keep the Strait of Hormuz open.

The U.S. moves outline the potential shape of a conflict between Iran and the West: Iran could rapidly mine the strait and use heavily armed speedboats to attack or ram Western ships trying to clear the waterway. A successful Iranian attack on a U.S. warship could drag America into a larger conflict.

Central Command officials have told lawmakers they want the new mine-detection systems fielded before this fall, according to defense officials, underlining the urgency of preparedness.

In addition, U.S. special-operations teams stationed in the United Arab Emirates would take part in any military action in the strait should Iran attempt to close it, defense officials said. A military official said these forces have been working to train elite local forces in Gulf nations including the U.A.E., Bahrain and Kuwait, but added: “They would be used in the event of active operations.”

According to defense officials, the Pentagon submitted a request to Congress on Feb. 7 on behalf of Central Command seeking to reallocate $100 million in defense funding to “bridge near-term capability gaps” in the Persian Gulf.

The request has yet to be made public because it is still being studied by lawmakers, defense officials said. The money will be used to upgrade patrol craft and unmanned drones, as well as to add small arms on surface ships, the officials said.

Congress was told the money was urgently needed, according to an official briefed on the plan. “You can buy it and deploy it rapidly,” the official said.

The new money comes on top of changes made last summer that provided Central Command with about $200 million for additional upgrades, some of which could be used in areas outside the Persian Gulf, defense officials said. The earlier request, which included money for a torpedo defense system, airborne antimine weapons and new cyber-weapons, was made by defense officials and backed without fanfare by Congress.

That request also included additional deployments of the SeaFox underwater drone, which is launched from a helicopter and uses a warhead to destroy mines. The system was deemed “an urgent operational need” by the U.S. Fifth Fleet, according to Navy officials.

The Pentagon and other U.S. agencies generally submit such reprogramming requests when they can’t wait until the next fiscal year. The Pentagon started making some adjustments as early as a year ago, but those didn’t require reprogramming.

The Pentagon told Congress that some of the new money would be used to modify existing weapons systems to be used against seaborne threats in the Persian Gulf and, specifically, the Strait of Hormuz.

Iran’s Islamic Revolutionary Guard deploys some of the fastest naval vessels in the Persian Gulf. These craft may be small—only 17 meters, or 56 feet, long in some cases—but they can carry machine guns, torpedoes and the Iranian-made “Kowsar” antiship cruise missile. Some can reach speeds of 60 to 70 knots, according to U.S. military intelligence analysts.

Antitank weapons are being reconfigured for use against swarms of these boats that could threaten U.S. warships, the Pentagon told Congress. Similarly, rapid-fire machine guns designed to shoot down missiles are being tested for use against small boats.

Pentagon war planners believe the addition of smaller-caliber guns would quickly make U.S. destroyers, which were designed mainly to fight other large ships, more effective against the Iranian craft.

“We are using capabilities we already have in a different way,” a senior defense official said.

The additional money for equipment upgrades is on top of the nearly $82 million the Pentagon sought in January to improve its largest conventional bunker-buster bomb, the 30,000-pound Massive Ordnance Penetrator.

The bomb, officials said, was designed to take out bunkers like those used by Iran to protect its most sensitive nuclear development work.

Western intelligence agencies had long suspected that the Iranian navy had between 2,000 and 3,000 mines, largely of Soviet or Chinese origin. But new intelligence suggests Iran may have as many as 5,000, including newer types that may be more powerful and harder to detect.

U.S. forces would also need to contend with Iran’s coastal air-defense system, shore-based artillery, Kilo-class and midget submarines, remote-controlled boats and unmanned kamikaze aerial vehicles, according to current and former U.S. officials.

The U.S. Navy has 14 minesweepers, three of which are stationed in Bahrain. Mackenzie Eaglen, a fellow at the American Enterprise Institute, said U.S. minesweeping capabilities have slipped because the military has deferred critical maintenance, a shortcoming it is “working overtime” to address.

—Keith Johnson and Jay Solomon contributed to this article.Write to Adam Entous at adam.entous@wsj.com and Julian E. Barnes at julian.barnes@wsj.com