Tehran's joint military command said the strait is now under "strict management and control of the armed forces" and accused the U.S. of "piracy" -- a reference to Washington's ongoing naval blockade of Iranian ports, which the IRGC says violates the terms of the ceasefire. Until that blockade is lifted, the strait stays shut.

The reversal came just hours after oil markets had already priced in the opening. Brent crude fell roughly 9.5% to around $89.89 a barrel Friday after Trump announced the strait was open. WTI slid more than 10% to $84.89. Those moves are likely to reverse when markets open.