DUBAI—Iran is using everything in its arsenal to try to stop the flow of oil leaving the Strait of Hormuz, resorting to guerrilla tactics to shore up the regime’s fast-eroding position in the conflict and pile pressure on the Trump administration in deadlocked negotiations.

Tehran has launched bomb-laden undersea drones, fired rocket-propelled grenades and deployed crews to attach limpet mines to ships over recent weeks, part of an asymmetric campaign targeting vessels that shuttle oil out of the Persian Gulf to waiting tankers, according to regional officials, security firms and analysts.

The U.S. military runs near-daily operations to help ships—and crude oil—get out of the strait while maintaining its blockade on Iran, an expensive effort that nevertheless recently returned flows to prewar levels.

The Islamic Republic’s battle to block that escape route has become a shadow war inside the wider U.S.-Iran conflict, with a direct impact on global energy prices as American forces have struck back. A spokesman for Iran’s foreign ministry didn’t respond to a request for comment.

On Monday, the U.S. attacked Iran’s Qeshm Island as part of a series of unannounced small-scale strikes in the waterway aimed at halting Iranian attacks on American assets and passing ships, The Wall Street Journal reported.

Benchmark Brent crude traded Tuesday around $102 a barrel, with the average gallon of gasoline costing about $4.15 as American voters cast ballots in the midterm elections. That is up from just over $70 a barrel before the war.

“They can keep it violent, they can keep risks to shipping high, they can keep costs high, they can keep oil prices high,” said Gregory Brew, Eurasia Group’s senior Iran and energy analyst.

The Trump administration insists that it has control of the strait, and the U.S. military in the region says the waterway is cleared of Iranian mines.

After flying over the strait last week, Adm. Brad Cooper, head of U.S. Central Command, said the U.S. maintained its patrols through the narrow waterway, which before the conflict was the conduit for 20% of the world’s oil supplies. “I didn’t see the IRGC out there, at all,” Cooper said, using an acronym for Iran’s Islamic Revolutionary Guard Corps.

October marked the highest monthly number of attacks in the strait since the war began, with more than 35 incidents logged by the U.K. Maritime Trade Operations Center, which is affiliated with the Royal Navy. At least five vessels have been struck so far in November, UKMTO said.

The attacks are designed to disrupt individual oil shipments through the strait and to re-establish the climate of fear that gripped shippers at the start of the conflict when Iran began to fire into the waterway and paralyzed shipbound trade in the region. A deal reached between Iran and the U.S. in June reopened the strait briefly, though Trump reinstated the U.S. blockade of Iranian ports as the two countries accused each other of violating the accord.

For weeks, the IRGC has claimed that mines laid by its forces in the strait have damaged commercial ships, according to Iranian state media. However, Gulf officials say those assertions are designed to sow panic among shippers and doubts about the Americans’ demining effort, with the ultimate aim of disrupting the flow of oil.

Since late August, Iran has used underwater, bomb-carrying sea drones to attack several commercial vessels, the officials said. Authorities in Tehran haven’t acknowledged the specific nature of those attacks, likely as part of the effort to suggest mines still litter the strait.

Iran has also resumed the use of limpet mines to disable vessels, a weapon it deployed in 2019, maritime experts say. On Friday, in the waters of the strait just north of Oman, a small boat approached the port side of a tanker, attaching two explosives to the side of the hull above the waterline. The subsequent blasts caused a fire in the engine room and disabled the ship temporarily, UKMTO said.

The U.S. Navy said Iran used limpet mines against ships off the United Arab Emirates in 2019, as tensions rose over President Trump’s withdrawal from the nuclear deal with Tehran.

The scope and location of Iran’s attacks has expanded as more oil has started leaving the strait, going beyond tankers to strike containerships, bulk carriers and gas carriers including those anchored outside the strait elsewhere in the Persian Gulf.

And, instead of relying on vulnerable coastal radars, which have been repeatedly targeted by U.S. airstrikes, Iran now favors human spotters, said Brew, the analyst.

Crew on small boats operating clandestinely among the fishing craft that crowd Hormuz can observe passing vessels and relay targeting information to missile and drone operators, a type of surveillance that is difficult for U.S. forces to detect and disrupt.

An Arab official familiar with Iranian attacks said such vessels often loitered around areas where attacks on commercial vessels occurred. Iran has seen much of its navy sunk by the Americans during the war, likely making the use of civilian vessels more necessary.

“Iran can inflict significant damage at little cost, while the ability to launch attacks from numerous coastal locations makes it harder and more time consuming for U.S. forces to detect and prevent them,” said Alessio Patalano, a maritime warfare expert at King’s College London.

The surge in attacks has coincided with signs that the recent recovery in shipping through Hormuz is faltering, although the latest oil-flow figures are preliminary and subject to upward revision.

According to ship tracker Kpler, oil flows through the strait, including associated ship-to-ship transfers, averaged around 7.8 million barrels a day in the week through Nov. 1, around 45% of prewar levels. Crude flows had reached close to prewar levels earlier in October.

The trade has become extraordinarily expensive: Shipping crude from the Gulf to Asia cost nearly $37 a barrel in mid-October, more than five times prewar levels, according to the International Energy Agency, a Paris-based intergovernmental energy watchdog.