DUBAI, United Arab Emirates (AP) — The United States is warning shipping companies that they could face sanctions for making payments to Iran to safely pass through the Strait of Hormuz.

The alert posted Friday by the U.S. Office of Foreign Assets Control adds another layer of pressure in the standoff between the U.S. and Iran over control of the Strait of Hormuz.

About a fifth of the world’s trade in oil and natural gas typically passes through the strait at the mouth of the Persian Gulf in peacetime.

Iran effectively closed the strait to normal traffic by attacking and threatening to attack ships after the U.S. and Israel launched a war on Feb. 28. It later began offering some ships safe passage by detouring them through alternate routes closer to its shoreline, charging fees at times for the service.