An Iranian flag flutters on board the Adrian Darya oil tanker, formerly known as Grace 1, off the coast of Gibraltar on August 18, 2019.

The U.S. State Department has offered millions of dollars to the captain of an Iranian oil tanker suspected of heading to Syria.

U.S. special representative to Iran, Brian Hook, reportedly emailed the captain of the Adrian Darya 1 about sailing the vessel to a country that would impound it on behalf of Washington.

The ship, which until recently was known as Grace 1, has been at the center of a diplomatic dispute between Iran and Western powers for several weeks.

The current whereabouts of the Iranian oil tanker is unknown, with reports suggesting it appears to have turned off its transponder in the Mediterranean west of Syria on Tuesday.