At least three Indian oil refiners and a global energy major plan to stop using vessels placed on Iran’s new blacklist, including for ship-to-ship (STS) transfers, due to security concerns, four sources familiar with the matter told Reuters.

STS transfers are operations in which cargo is moved directly from one vessel to another, often at sea or in designated offshore areas, without the ships docking at a port.

At the beginning of the week, Iran blacklisted 45 vessels it said had violated its rules for crossing the Strait of Hormuz and warned that action would be taken against vessels conducting cargo transfers with the listed ships.

The move escalates Tehran’s pressure on shipping through the strategic waterway, which is a key route for global energy supplies, amid the ongoing U.S.-Israeli war on Iran.

The blacklist appears aimed at disrupting so-called shuttle runs conducted by Gulf oil producers, including the UAE and Saudi Arabia, which use dedicated tankers to transport crude through the Strait of Hormuz before transferring the cargo via STS operations in the Gulf of Oman to vessels bound for end-users.

These shuttle operations have helped maintain Middle Eastern oil flows disrupted by Iran’s restrictions on shipping through the Strait amid the conflict.

The Persian Gulf Strait Authority, a newly established Iranian body responsible for managing the waterway, said the blacklisted vessels could face fines, detention and cargo confiscation.

The sources said the Indian refiners and global energy major were moving to avoid the listed vessels as concerns grow over the potential security and operational risks associated with using them for oil transportation and STS transfers.