The United States is warning shipping companies they could face sanctions for making payments to Iran to safely pass through the Strait of Hormuz. The U.S. Office of Foreign Assets Control alert posted Friday adds pressure in the standoff between the U.S. and Iran over control of the strait at the mouth of the Persian Gulf , where about a fifth of the world's trade in oil and natural gas typically passes. Iran effectively closed the strait by attacking and threatening 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 routes closer to its shore, charging fees at times. That “ tollbooth ” effort is the focus of the U.S. sanctions warning, which said payment demands could include transfers not only in cash but also “digital assets, offsets, informal swaps, or other in-kind payments,” including charitable donations and payments at Iranian embassies. The U.S.…