The US Treasury Department warned in a separate statement against paying a "toll" to Iran's government in exchange for safe passage through the Strait of Hormuz, saying this could also trigger sanctions.

Tehran has virtually blocked the Strait of Hormuz, a key waterway for energy transit, since US-Israeli strikes on Iran from late February.

This has caused oil prices to surge globally and US gasoline prices to spike, putting pressure on American households.

US Treasury Secretary Scott Bessent said Friday that his agency would "relentlessly target the regime's ability to generate, move, and repatriate funds, and pursue anyone enabling Tehran's attempts to evade sanctions."

In targeting the foreign currency exchange firms, the US Treasury charged that Iran's "shadow banking networks handle tens of billions of dollars' worth of trade each year, much of it derived from Iran's overseas sales of oil and petrochemicals."

In a separate statement, the Treasury's Office of Foreign Assets Control issued an alert to warn people from and outside the United States about sanctions risks of making payments to Iran's government for safe passage through the Strait of Hormuz.