The US military on Monday detailed the boundaries of its Strait of Hormuz blockade, saying it would extend east to the Gulf of Oman and Arabian Sea, while ship-tracking data showed two ships turned around in the strait as the blockade went into effect.

In a note to seafarers about the blockade that seeks to take control of the strait away from Iran, the US Central Command said: “Any vessel entering or departing the blockaded area without authorization is subject to interception, diversion, and capture.”

“The blockade will not impede neutral transit passage through the Strait of Hormuz to or from non-Iranian destinations.”

The blockade began at 10 a.m. US Eastern Time on Monday.

Data from the MarineTraffic tracking service showed the tanker Rich Starry, which departed Sharjah anchorage off the coast of Dubai on Monday heading for China, turned back minutes after approaching the strait as did a second vessel, the Ostria. Those tankers can carry oil and chemicals.

However, two oil tankers linked to Iran exited the Gulf on Monday via the strait ahead of the planned US blockade, shipping data from Kpler and LSEG showed.

The tanker Aurora is laden with Iranian oil products, while the New Future tanker is carrying diesel loaded from the Hamriyah port in the United Arab Emirates, Kpler data showed.

They were among at least eight ships that transited the strait on Monday before the blockade.

According to Kpler, an oil products tanker that loaded in a United Arab Emirates port and three dry bulk ships that left from Iranian ports exited the strait while two Pakistani oil products tankers and two dry bulk ships entered the strait.

US President Donald Trump announced the blockade following the breakdown of weekend talks to end the six-week-long war between the US and Iran, sending oil prices back above $100 a barrel.

The blockade adds to uncertainty around how ships will transit the crucial waterway, used to move one-fifth of the world’s oil and gas supplies, with experts saying that enforcing it will require a sustained commitment of US Navy ships and personnel, as well as clear guidance from the Trump administration and the Navy’s legal department.

The US has 16 warships in the Middle East, a defense official said. A second defense official said no warships are in the Persian Gulf — the body of water that forms most of Iran’s coastline. Both spoke on condition of anonymity to discuss sensitive military operations.

The second official pointed to a notice to mariners as a more accurate representation of the military’s plans. It says access to Iranian ports is being restricted, but the ways these measures “will be applied in practice … are in development.”

According to US officials quoted by the Wall Street Journal, the American military is expected to attempt to intercept or quarantine commercial ships on either side of the Strait of Hormuz to block them from Iranian ports, requiring special forces such as the Navy SEALs to deal with vessels that refuse requests to board.

The US military note said the blockade would include all of Iran’s coastline but humanitarian shipments including food, medical supplies, and other essential goods would be permitted, subject to inspection.

“The immediate effect will be a sharp drop in fresh fixture activity for any voyage with Iran exposure, and a broader hesitation even for non-Iran cargoes moving near the Strait,” said Shohruh Zukhritdinov, a Dubai-based oil trader.

International Maritime Organization Secretary-General Arsenio Dominguez told reporters the US blockade would extend a halt to ship traffic in the strait since the conflict began. Dominguez said 20,000 seafarers and around 1,600 vessels were stuck in the Gulf.

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