What to know about the US sea blockade on Iran

The U.S. Navy’s blockade against Iran is showing early signs of effectiveness, as vessels linked to Iran have been forced to halt or reverse course after attempting to leave the Persian Gulf. According to Admiral Brad Cooper, head of U.S. Central Command, the blockade that began Monday “has been fully implemented” and has “completely halted economic trade going in and out of Iran by sea.”

This maritime enforcement action represents a significant escalation in tensions and could place substantial economic pressure on Iran’s economy. It comes in response to Iran’s earlier disruption of shipping through the strategically vital Strait of Hormuz, which has sent global energy prices climbing amid the ongoing conflict.

“U.S. forces have completely halted economic trade going in and out of Iran by sea,” Cooper stated. The blockade is being enforced beyond the Strait of Hormuz in the Gulf of Oman, with the U.S. military observing vessels departing Iranian facilities before intercepting them once they’ve cleared the strait.

The U.S. Central Command reported Wednesday that no vessels had managed to bypass the blockade during its first 48 hours of operation. Ten vessels have already complied with directions to turn around and return toward Iranian ports or coastal areas. Navy warships are communicating directly with merchant ships, making it clear they are prepared to board vessels and use force if necessary to ensure compliance.

Shipping data confirms the blockade’s impact. Ana Subasic, trade risk analyst at data firm Kpler, noted that on Tuesday, only eight vessels—most linked to Iran or under sanctions—transited the strait. “Most of the vessels have appeared to halt or have reduced movement after clearing the strait,” she said, “which tells us that the effect of the blockade is starting to show up.”

Maritime tracking data reveals several vessels attempting to evade detection or changing course after encountering the blockade. The Rich Starry, a Chinese-owned tanker previously sanctioned by the U.S. for smuggling Iranian petroleum products, entered the Persian Gulf empty on April 4, turned off its transponder for over a week—a common tactic called “running dark”—then reappeared laden with oil before abruptly reversing course after passing through the strait.

Similarly, the oil-products tanker Elpis departed Iranian waters Monday, passed through the strait, then cut its engines in the Gulf of Oman before disabling its radio transponder on Tuesday.

Maritime intelligence firm Windward described vessel behavior as “indicating a fragmented and uneven response to the blockade” as sanctioned vessels continued to be active, with some transiting the strait while others delayed or reversed course.

The U.S. blockade comes as a direct response to Iran’s effective closure of the Strait of Hormuz through threats against shipping. Iran’s actions had blocked approximately 20% of the world’s typical daily oil consumption, causing oil prices to surge and prompting warnings about higher inflation and possible recessions in major economies. Vessels had been attacked with aerial and undersea drones and unknown projectiles, killing 11 crew members, and causing ship traffic to drop by more than 90%.

While some oil from Gulf producing nations has been rerouted through pipelines to the Red Sea and Gulf of Oman, these alternative routes cannot compensate for the effective closure of the strait. Iran has also begun vetting the few vessels still attempting passage, requiring detailed information on cargo and crew to be submitted to the Revolutionary Guard and imposing a $1 fee per barrel of oil or fuel products.

The terms of the U.S. blockade allow for exceptions, creating some uncertainty in maritime operations. According to notices to mariners, the blockade permits “humanitarian shipments including food and medical supplies essential for the survival of the civilian populations” to pass with inspections, aligning with international law on naval warfare. “Neutral” ships may also pass after inspection, though the definition of “neutral” remains unclear.

Iran has responded with threats of its own. On Wednesday, the commander of Iran’s joint military command warned that Iran would completely block exports and imports across the entire Persian Gulf region, Sea of Oman, and Red Sea if the U.S. does not lift its blockade. “Iran will act with strength to defend its national sovereignty and its interests,” Ali Abdollahi stated, adding that the U.S. blockade represents “a prelude to violating the ceasefire.”

The standoff carries significant implications for Iran’s economy. Without the ability to export oil, the country’s storage facilities will eventually fill, forcing it to shut down wells that are difficult to restart. Additionally, Iran relies on gasoline imports due to insufficient domestic refinery capacity.