The U.S. is preparing to board and seize Iran-linked oil tankers and other commercial ships in international waters in the coming days, expanding its pressure campaign on Tehran.

U.S. officials said the operation would target Iranian-flagged vessels and “dark fleet” ships used to move sanctioned Iranian oil to pressure it to fully reopen the Strait of Hormuz to normal shipping.

Iran escalated this weekend by tightening its control over the Strait of Hormuz, attacking commercial vessels and warning that ships trying to cross could be destroyed.

Tehran said it was doing so because the U.S. had not guaranteed free passage for ships traveling to and from Iran, and said it would keep the strait under tight control until that changed.