The United States has warned shipping companies they could face sanctions if they make payments to Iran for safe passage through the Strait of Hormuz, escalating tensions over control of the vital global energy route.

The warning was issued Friday in an alert by the US Office of Foreign Assets Control (OFAC), as Washington and Tehran remain locked in a standoff over the strategic waterway at the mouth of the Persian Gulf. Roughly one-fifth of the world’s oil and natural gas trade normally passes through the strait.

According to the alert, Iran has demanded fees from some vessels in exchange for secure transit after rerouting ships through lanes closer to its coastline. The US described the practice as a “tollbooth” scheme.

Washington said such payments could include not only cash transfers, but also digital assets, offsets, informal swaps, in-kind transactions, charitable donations or payments made through Iranian embassies.

Iran moved to restrict traffic in the strait after the United States and Israel launched military operations on Feb. 28, attacking and threatening commercial shipping, according to reports.

In response, the US imposed a naval blockade on Iranian ports on April 13, preventing Iranian tankers from departing and cutting off a major source of oil revenue for Tehran.

The US Central Command said 45 commercial ships had been instructed to turn back since the blockade began.