Satellites have portrayed the Iranian oil tanker Adrian Darya 1, the focus of a row between Tehran and Western countries, off the Syrian port of Tartous, according to US space technology company Maxar Technologies.
The Associated Press reported that the tanker was spotted off Syria, despite Iran's pledge not to go to it when the Gibraltar authorities agreed to release the tanker weeks ago.
The vessel's apparent shutdown of the transceiver in the Mediterranean off the west coast of Syria, the ship's tracking data showed on Tuesday.
The tanker, carrying Iranian oil, sent the last sign of its position between Cyprus and Syria, sailing north on Monday noon, the data showed.
US National Security Adviser John Bolton said: "Anyone who says Adrian Daria 1 is not going to Syria is more important."
"Tehran believes that financing the murderous Assad regime is more important than taking into account the interest of its people," he said in a tweet on Twitter. "We can talk, but none of the sanctions against Iran will be eased before it stops lying and spreading terrorism."
The British Special Marines detained the tanker, previously known as Grace 1, off Gibraltar on July 4 on suspicion of heading to Syria in violation of EU sanctions.
Two weeks later, Iran seized a British-flagged tanker in the Strait of Hormuz.
The Gibraltar government released the Iranian tanker on August 15 after receiving official written assurances from Tehran that the tanker would not unload its 2.1 million barrels of oil, worth about $ 130 million, in Syria.
However, shipping sources are likely to try to move part of its cargo to another vessel after Iran said the sale was complete.
Washington has placed the tanker on the sanctions list for providing support to the Revolutionary Guards on the US sanctions list, according to a statement from the US Treasury Department.