WASHINGTON — U.S. forces struck Iranian rocket launchers on the Strait of Hormuz in their first military action in a month on Sunday, breaking a lull in fighting during an intermittent war that has lasted more than six months. Iran swiftly vowed to retaliate for what it called a deadly attack.

Forces with the Revolutionary Guard Corps were observed preparing to launch rockets with sea mines into the strait, according to Tim Hawkins, a spokesperson for the U.S. Central Command. The U.S. military last week completed clearing sea mines from the strait’s international shipping routes.

Semiofficial news outlets in Iran reported sounds of explosions near Larak island on the strait. The Guard in a statement carried by Iran's state broadcaster noted “the martyrdom and injury of several of our fighters and compatriots.”

A return to open conflict would be dangerous for the region, where Iran has targeted U.S. military bases and infrastructure in Gulf countries since the war began on Feb. 28.

The last time the U.S. military confirmed targeting Iran was on July 29, when it announced a "heavy wave of strikes" on dozens of Revolutionary Guard targets including coastal surveillance and defense sites. On Aug. 1, President Donald Trump said he would order the military to hold off on new strikes at the urging of Gulf allies Qatar, Saudi Arabia and the United Arab Emirates.

The US had turned to economic pressure

The attack comes just days after the Trump administration said it would turn its focus to increasing economic pressure — rather than military action — to try to end its war with Iran. The shifting strategy centers on threats to punish any country or entity that continues to conduct business with Tehran.

Sunday's attack was a “fatal mistake by the Trump regime during the economic war” and the enemy will pay the consequences militarily and economically, a Guard spokesperson, Gen. Hossein Mohebi, said in a statement carried by Iranian media.

The U.S. turn to using sanctions came as the administration weighs diminished munitions stockpiles after months of war, sparking concerns that the prolonged conflict could undermine U.S. military readiness in other parts of the globe.

As the conflict grinds on, Trump's talk of finding a quick end to the war also appears to be fading. He stressed last week that he's "not in a hurry" to get Iran back to the negotiating table, and he continues making the case that the Islamic Republic's leadership is on the ropes.

Appointments to Iran's senior security leadership in the past month, however, have signaled Tehran's defiance after weathering decades of sanctions as well as a lack of tolerance for dissent at home.

Trump has consistently emphasized that the U.S. and Israel campaign has been devastating for Iran's navy and air force. Iranian officials have said the country has suffered $270 billion in direct and indirect damage. Israeli military strikes in the first weeks of the war wiped out much of the theocratic government's leadership structure.

Iran leans on the strait for leverage

But Iran has found leverage through its strikes against vessels on the Strait of Hormuz, where relatively few ships carrying oil and liquefied natural gas are risking passage. Iran still has enough drones and missiles to fire at vessels transiting the waterway through which 20% of the world's oil normally flows.

Crude oil prices rose Sunday in response to the U.S. attack. The price for a barrel of U.S. oil went up 1.8% to $84.94, while the price for a barrel of Brent crude, the international standard, rose 1.9% to $89.79.

Tehran continues to assert that even after it works out a management plan for the strait with Oman, its neighbor on the other side of the waterway, the U.S. can't use it until it fulfills its obligations under the memorandum of understanding the countries signed in mid-June.

Trump has repeatedly declared that the “Strait of Hormuz is open,” saying 24 vessels passed through last week. But that’s a fraction of the roughly 130 vessels that transited daily before the war began.

Earlier on Sunday, a multinational coalition overseen by the U.S. Navy said commercial traffic through the strait remained at “reduced levels.”

And a monitoring agency run by the British military said an unknown projectile struck a tanker ship on Saturday north of Khasab, Oman, in the strait. The United Kingdom Maritime Trade Organization said no casualties or environmental impact had been reported, and cited unspecified military authorities. There was no immediate claim of responsibility.

The U.S. military said that as of Sunday, it had redirected 83 commercial vessels and disabled three under its blockade of Iranian ports.

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Toropin reported from Nuremberg, Germany. Anna reported from Lowville, New York. Associated Press journalists Christopher Weber in Los Angeles and Dee-Ann Durbin in Detroit contributed to this report.

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