A quartet of Iran-linked tankers changed course in waters just outside the Strait of Hormuz, evidence that a reimposed U.S. blockade on the Islamic Republic’s shipping is once again squeezing Tehran.

Three U.S.-sanctioned fuel tankers U-turned, diverted or zig-zagged in the Gulf of Oman and Arabian Sea on July 16, ship tracking shows. A fourth vessel, an oil supertanker, turned back when it was sailing toward the waterway from the Arabian Sea. At the same time, one non-Iranian oil carrier may have attempted in-bound transit through the strait.

The U.S. restored a blockade against Iranian shipping on July 14, a step that all but halted Tehran’s crude exports when it was imposed previously. At the same time, Iran has increasingly been targeting oil supertankers as they pass through a stretch of Hormuz where U.S. forces have been coordinating transits. Combined, the moves are adding to the risks of getting oil out of the Persian Gulf, or sailing empty ships in to collect new cargoes.

Since imposing the latest blockade, U.S. forces have redirected three merchant ships, boarded a vessel for verification purposes, and disabled a tanker that failed to comply with instructions, U.S. Central Command said in a social-media post late on July 16.

The amount of observable movements — in which ships keep their digital location beacons turned on — has collapsed as ships do everything possible to stay safe. One oil supertanker appeared to be moving north toward Hormuz late on July 16, and has since turned off its signal, suggesting it may have attempted overnight transit.

The locations of the disabled tanker, which was hit by U.S. missiles deep in the Persian Gulf near Iran’s oil export terminal of Kharg Island, and the ship that was boarded, in the Gulf of Oman, point to a wider area of operations for the new blockade compared with the previous one. 

The very large crude carrier Vivian, also known as Titan, turned back on July 15, when it had been sailing toward the Persian Gulf, and continued sailing away on July 17. The ship is under U.S. sanctions for its role in helping Iran.

The liquefied petroleum gas carrier Glendale was sailing in the Gulf of Oman at a steady pace into the Arabian Sea late on July 16 when it abruptly made a U-turn, and came to a stop off the coast of Oman on July 17, according to ship-tracking data. Danuta I was sailing toward Sri Lanka, but had barely made it into the Arabian Sea on July 16 when it began moving in an unusual zig-zag fashion. It’s now traveling at a slow speed that’s not in the direction of Sri Lanka.

A third U.S.-sanctioned LPG vessel, the very large gas carrier Celeste, was observed sailing into the Arabian Sea, and indicating China as its destination.

Some 91 Iran-linked oil, LPG and petrochemical tankers — including empty ones — have been spotted in satellite images in the Persian Gulf and the Gulf of Oman over the past seven days, according to figures from nonprofit organization United Against Nuclear Iran. The energy and shipping markets are keenly following the movements of the flotilla to understand how tightly the U.S. military is running its blockade.

Elsewhere, Iran-linked ships that had left the Persian Gulf earlier appeared to also make abrupt changes to planned journeys.

This week, two that had signaled Karachi as their destination later reversed course back toward the Gulf of Oman as the blockade came into effect. A third tanker was sailing westward past Sri Lanka before making a U-turn on July 16, and is now anchored off the country’s coast.

Observable shipping activity through the Strait of Hormuz remained subdued early on July 17. Only four Iran-linked vessels crossed in either direction with their transponders turned on the previous day, although one small oil tanker that had made an eastbound transit to leave the Persian Gulf then turned around again to return to the inland sea.

Glendale’s owner is listed as Ecoseas Maritime Ltd., which shares an India-based address with its manager, Bluveera Shipping OPC Pvt Ltd., on the Equasis database. A call made to Ecoseas’s Bulgarian telephone number didn’t get through, while Bluveera has no email or contact number listed. Similarly, no emails or phone numbers were listed for Danuta I’s owner and manager, Panama-based Ithaki Maritime and Trading SA. Celeste’s owner, Aerilyn Shipping Inc., also based in Panama, didn’t list any email or phone number.

An email to the commercial manager of the Vivian wasn’t immediately answered.