The seizure of the St. Nikolas, earlier on Thursday, is at the center of a major dispute between Washington and Tehran, and may likely stoke regional tensions.
The tanker was carrying Iraqi crude destined for Turkey.
Vessel tracking software shows its course - the tanker turned off its AIS tracking system as it turned toward an Iranian port.
U.S. authorities seized the St. Nikolas last year in a sanctions operation, and when it had sailed under a different name, Sueze Rajan.
Tehran says its action Thursday was in retaliation for that earlier U.S. confiscation.
The seizure coincides with weeks of attacks by Yemen's Iran-backed Houthi militias targeting Red Sea shipping routes.
White House national security spokesperson John Kirby condemned Iran's actions on Thursday.
"The Iranian government should immediately release the ship and its crew. These provocative and unacceptable actions need to stop. We'll continue to work with our allies and partners to deter and confront the full range of Iran's concerning and destabilizing behavior in close coordination, of course, with the international community"
