DUBAI-A new wave of attacks on vessels around the Strait of Hormuz is threatening a recovery in Middle Eastern oil exports-just as Gulf producers have pushed crude shipments back toward prewar levels.

The swift rebound in shipments in recent weeks has been driven by U.S. naval protection helped by earlier waves of strikes that knocked out Iranian radar and communications along the contested strait. The United Arab Emirates and other producers have set up an elaborate system of shuttle runs in which tankers load inside the Persian Gulf, exit Hormuz and transfer the crude to vessels waiting outside the waterway.

But after several days without reporting attacks on ships toward the end of last month, the U.K. Maritime Trade Operations, which is affiliated with the Royal Navy, has reported seven strikes on vessels around the waterway since Sept. 28. On Sunday, it reported another strike, but didn't say when the incident happened.

Iran's Islamic Revolutionary Guard Corps navy issued a warning to ships planning to use the U.S.-backed route through the strait on Saturday, saying that they would be targeted and destroyed. "Don't trust U.S. Navy and don't use south corridor at all and don't put your life in danger," a radio message transmitted on a public shipping channel and reviewed by The Wall Street Journal said.

The renewed attacks may already be taking a toll, with early signs that oil flows are beginning to slow, analysts say. Firm data comes with a lag, because many ships are operating without their transponders switched on to avoid becoming targets.

All seven vessels were attacked near the narrowest part of the Hormuz strait, according to UKMTO.

Iran's ability to target ships has also improved in recent weeks, adding to the risks in the strait, according to a U.S. official. The U.S. several weeks ago said it would strike Iranian oil tankers in response to Tehran's attacks, but it has since backed off that effort after Iran unleashed a wave of missiles at a U.S. air base in Jordan, the official said.

The new strikes expose the fragility of the recovery in oil flows and come as President Trump, who has claimed full U.S. control over Hormuz, weighs renewed military action against Iran.

Any fresh disruption would hit a global economy already grappling with high energy and rising borrowing costs. International benchmark oil prices remain around $100 a barrel despite recovering exports, reflecting fears that renewed attacks could again curb flows and keep freight and security costs elevated.

"The step-up in attacks on ships highlights how the current equilibrium in the oil market is fragile and could easily be shattered," said Hamad Hussain, a senior economist at Capital Economics. "This would especially be the case if there is further escalation and energy infrastructure is targeted."

Brent, the international oil benchmark, settled Friday at $102.25 a barrel, up nearly 5% for the week. Hussain said traders were pricing in a substantial risk that flows would fall again. Capital Economics expects Brent to remain around $100 through the end of the year.

Among the seven vessels attacked in the past week, at least four had completed two or more roundtrips in and out of Strait of Hormuz since June, carrying crude oil or oil products, according to ship tracker Kpler.

Al Funtas, a so-called very large crude carrier operated by Kuwait Oil Tanker Company, was completing its fifth shuttle run between the Port of Mina Al-Ahmadi in Kuwait and ports in the Gulf of Oman when it was attacked on Sept. 28, according to Kpler. Kazimah III, another VLCC operated by KOTC, had carried out three shuttle runs and was on its way into the Persian Gulf when it was hit on Oct. 1.

Ships of that size can carry as many as two million barrels of oil at a time.

Since the U.S. and Israeli strikes that began the war in February, Iran has used attacks on ships to throttle shipping through Hormuz, once the thoroughfare for a fifth of the world's oil. That has driven up the price of gasoline and diesel for consumers around the world and has forced Saudi Arabia and the U.A.E. to divert exports around the strait through bypass pipelines.

Kpler said regional crude exports excluding Iran averaged at least 16.5 million barrels a day from Sept. 1 through Sept. 28, close to prewar levels.

But there are signs Iran's recent attacks may already be starting to slow that recovery.

Rory Johnston, founder of oil research firm Commodity Context, said he has seen a potential pullback in recent days and estimates flows are down by two million to three million barrels a day, though those initial totals could be revised higher as better information becomes available.

"The recent pace of flows, while impressive, has never been sustainable and has already come at great expense," Johnston said. Part of the surge may reflect exporters rushing barrels out before a feared new escalation, he said. Gulf oil producers are shelling out between $30 million to $40 million for a round trip shuttle run in and out of Hormuz-or $15 to $20 a barrel, according to shipbrokers, and that figure doesn't include insurance costs.

Saudi Arabia had shifted exports back through Hormuz after a Sept. 10 attack on its East-West pipeline and attacks on its ships by Yemen's Houthi militants interrupted Red Sea shipments. The pipeline remains below capacity, but renewed loading at the Yanbu port on the Red Sea could help offset another Hormuz slowdown, Hussain said.

Standard Chartered estimates that total Saudi exports rebounded to 6.9 million barrels a day in September from 2.45 million in August.

But restored volumes don't mean a return to normal.

In September, most of the crude that crossed the Strait of Hormuz went via tankers transferring their oil to ships waiting off the coast of the U.A.E. and Oman, according to Kpler.

The costs of such shuttle runs and military-protected convoys may not be sustainable, wrote Kim Fustier, senior global oil and gas analyst at HSBC. Meanwhile, margins for diesel and other refined products remain high.

"All of this suggests that Middle East exports-at least for crude-are rising, but at enormous cost," Fustier wrote.