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The U.S. Navy is imposing a sea blockade against Iran — the latest escalation in the war that has seen the flow of key energy supplies choked off at the Strait of Hormuz.

The blockade that started Monday “has been fully implemented,” according to a statement Wednesday from Adm. Brad Cooper, commander of the U.S. Central Command. “U.S. forces have completely halted economic trade going in and out of Iran by sea,” he said. The blockade could put serious pressure on the Iranian economy.

Shipping data firms say Iran-linked or sanctioned vessels have left the Gulf through the Strait — but then stopped or turned around. An uncertain and risky shipping situation is being complicated by jamming or faking of location systems.

Here are key things to know about the blockade and the situation at the Strait of Hormuz.

US Navy enforcing blockade

The blockade is being enforced “impartially against all vessels of all nations entering or leaving coastal areas or ports in Iran.” Freedom of navigation remains for vessels not entering Iranian ports, Central Command said.

The U.S. military will operate the blockade in the Gulf of Oman beyond the strait, a U.S. official told The Associated Press. The U.S. official, who spoke on condition of anonymity to discuss sensitive military operations, explained that the strategy is to observe vessels subject to the blockade leave Iranian facilities and clear the Strait of Hormuz before intercepting them and forcing them to turn around.

The official said that the military relies on more than just automated tracking beacons that all merchant ships are required to carry, called AIS, to determine merchant ships were coming from a port in Iran, but wouldn’t go into more detail citing the need for operational security.

Ships are turning around as traffic adjusts

U.S. Central Command said in a statement Wednesday that no vessels have made it past U.S. naval forces during the first 48 hours of the blockade on ships entering and exiting Iranian ports.

Central Command said that nine vessels have complied with directions from U.S. forces to turn around and return toward an Iranian port or Iran’s coastal area. Navy warships are telling merchant ships that they are ready to board them and use force to compel compliance.

On Tuesday, the first full day of the blockade, only eight vessels, most of them linked to Iran or sanctioned, transited the strait, said Ana Subasic, trade risk analyst at data and analytical firm Kpler. The operating environment is still considered “extremely high risk” despite the current ceasefire, she said.

"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 because most of these vessels that have crossed have some kind of history with carrying Iranian-origin sanctioned cargo."

One sanctioned vessel, the Rich Starry tanker carrying methanol, left the Persian Gulf through the strait but then halted, did a U-turn and went back through the strait, she said.

Maritime intelligence firm Windward said that vessel behavior was “indicating a fragmented and uneven response to the blockade” as sanctioned and false-flagged vessels continued to be active, some transiting the strait, others delaying or reversing course.

Trying to break Iran’s chokehold

Iran had blocked the Strait through the threat of attack on shipping, cutting off 20% of the world’s daily oil consumption, sending oil prices sharply higher and leading to warnings about higher inflation and recessions in leading economies.

Vessels were hit with aerial and undersea drones as well as unknown projectiles, killing 11 crew members. While those attacks have dwindled, the risk of navigating the area means that ship traffic has dropped by more than 90%.

Some of that blocked oil is making it out from Gulf producing nations through pipelines to the Red Sea and the Gulf of Oman. But those pipelines can’t make up for the effective closure of the Strait.

Iran has also started vetting and collecting money from the few vessels daring to pass the strait. Vessels must submit detailed information on cargo and crew to the paramilitary Islamic Revolutionary Guards Corps and pay a $1 fee per barrel of oil or fuel products before being allowed to pass, according to Kpler.

U.S. and Israeli vessels are barred while some vessels from Iran, India and China have made it through. India has intervened diplomatically to get its vessels out.

The White House has demanded that Iran reopen the strait.

Blockading the coast puts serious pressure on the Iranian economy, especially its oil industry. Unless it can export oil, available storage will fill up and it will have to shut down wells that are difficult to restart. Additionally, Iran imports gasoline since it lacks the refinery capacity to turn its own oil into fuel.

The US blockade has a rule book

The terms of the U.S. naval blockade of the Iranian coast have contributed to difficulty for outsiders of ascertaining the state of ship traffic. According to the notice to mariners sent out to ship, the blockade is being enforced in the Gulf of Oman and the Arabian Sea, not at the Strait of Hormuz. So simply passing the strait doesn't mean a vessel beat the blockade.

Additionally, there was a grace period extending until 1400 GMT on Monday for ships to leave Iranian ports. And “humanitarian shipments including food and medical supplies essential for the survival of the civilian populations” will be permitted to pass subject to inspections.

That last provision is in accordance with international law on naval warfare, which permits blockades but bars those that are solely intended to starve civilians, according to a legal guide from the U.S. Naval War College cited by maritime historian Sal Mercogliano, who runs a YouTube channel on shipping.

"Neutral” ships can pass, though they may be inspected, but it’s not clear what “neutral” means. Lloyd’s List Intelligence maritime data firm said the U.S. move “has plunged shipowners into fresh uncertainty around enforcement.”

So all that means that ships from Iranian ports can be detected passing the Strait — and still face the risk of being stopped farther out. Or not, if they left before the end of the grace period.

Meanwhile container ships seen heading for Iranian ports could be allowed in or out if they’re carrying food, or not, if they’re carrying other goods.

Meanwhile ships can broadcast false information on their AIS systems used to avoid collisions, or turn the systems off.

Iran would block Gulf trade if blockade continues

The commander of Iran’s joint military command warned Wednesday that Iran would completely block exports and imports across the Persian Gulf region, the Sea of Oman and the Red Sea if the U.S. does not lift its blockade on Iranian ports.

“Iran will act with strength to defend its national sovereignty and its interests,” said Ali Abdollahi. He added that the U.S. blockade is “a prelude to violating the ceasefire.”

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Toropin reported from Washington, DC