Forces with the Revolutionary Guard Corps were observed preparing to launch rockets with sea mines into the strait, according to the official, who spoke on condition of anonymity to detail sensitive military movements.

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.”

The statement said the attack will “result in punishment of the aggressor.”

The fighting 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.

The turn to using sanctions as the cudgel of choice comes 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.

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.

But Iran has found leverage through its own strikes in the critical Strait of Hormuz where relatively few vessels carrying oil and liquefied natural gas are risking passage. Iran still has enough drones and missiles to fire at vessels transiting the vital energy waterway, through which 20% of the world’s oil normally flows, effectively controlling much of the traffic in the strait.

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 passed through the vital waterway 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 the ship had been moving inbound and no casualties or environmental impact had been reported. It cited unspecified military authorities. There was no immediate claim of responsibility.