The Iranian oil tanker at the centre of a diplomatic dispute departed from Gibraltar late on Sunday after the British overseas territory rejected a US demand to seize the vessel.
According to the monitoring website Marine Traffic, the supertanker which had been detained since July 4 off the coast of Gibraltar lifted anchor on Sunday evening before 2300 GMT and started sailing south.
Al Jazeera's Andrew Simmons, reporting from Gibraltar, has also confirmed that the ship has left and is "heading east and is about to join the international shipping lanes in the Strait of Gibraltar."
"Where it is heading to, destination unknown. It just says on the transponder, Mediterranean. That appears to fit with the tracking so far."
He also reported hearing radio communication from authorities in Gibraltar and the ship confirming its departure.
"This had been a long affair, the preparation for the ship. We saw the loading of supplies several hours before darkness, and the boarding of the ship using a special safe basket to bring up a technician to fix something, and at least five other crew members."
The technician left four or five hours later and headed back to shore, he added.
The ship now renamed Adrian Darya 1 was previously known as Grace 1. It has a cargo of an estimated US$140m worth of light crude oil.
"It is now complying with international maritime law having been re-flagged and renamed on all parts of this vessel," Al Jazeera's Simmons said.
Gibraltar's Supreme Court on Thursday ordered the release of the vessel after more than 40 days in detention.
The decision came after Gibraltar's government said it had received written assurances from Iran that the ship would not be headed for countries "subject to European Union sanctions".
British Royal Marines had seized the vessel in Gibraltar in July on suspicion that it was carrying oil to Syria, a close ally of Iran, in violation of European Union sanctions.
That detention ended last week, but on Friday a US court issued a warrant for the seizure of the tanker, on the grounds that it had links to Iran's Islamic Revolutionary Guard Corps (IRGC), which Washington has designated a "terrorist" organisation.
Gibraltar's government said on Sunday it could not comply with the US request because of European law.
"The Central Authority's inability to seek the Orders requested is a result of the operation of European Union law and the differences in the sanctions regimes applicable to Iran in the EU and the US," the government said in a statement.
"The EU sanctions regime against Iran - which is applicable in Gibraltar - is much narrower than that applicable in the US."
Iran has denied the tanker was ever headed to Syria.
The seizure triggered a sharp deterioration in relations between Iran and the United Kingdom. Tehran subsequently detained the British-flagged tanker in what was seen as a tit-for-tat move.
That tanker, the Stena Impero, is still in Iranian custody.
SOURCE: Al Jazeera and news agencies