It was eventually released after assurances were given that it would not head for the war-ravaged country.
But images released on Saturday seemed to show it two nautical miles offshore, media reports said.
The images -- from US company Maxar Technologies -- appeared to place the tanker very close to the Syrian port of Tartus on September 6.
US National Security Adviser John Bolton tweeted that anyone who believed the ship was no longer headed for Syria was "in denial".
"Tehran thinks it's more important to fund the murderous Assad regime than provide for its own people," he said, alongside another satellite picture. "We can talk, but #Iran's not getting any sanctions relief until it stops lying and spreading terror!"
There is however no confirmation that the ship was unloading its cargo of 2.1 million barrels of Iranian crude oil. Neither Iran nor Syria commented, the BBC reported.
The seizure of the ship off the British territory had caused a major diplomatic spat between Washington and Tehran. British marines had helped Gibraltar authorities detain the vessel, partly drawing the UK into the row.
Later, the US made an official request to seize the ship in August, but the courts in Gibraltar denied it.
The Gibraltar authorities freed the vessel on August 15 after receiving assurances from Iran that it would not discharge its cargo in Syria.
The US has been seeking to seize the tanker since it was released by Gibraltar. It issued a warrant and blacklisted the vessel, threatening sanctions on any country which offered it aid.
The ship has since been sailing east across the Mediterranean.