Adrian Darya 1 has been zigzagging since it was released from Gibraltar on August 18.

The Iranian tanker Adrian Darya 1 blacklisted by the United States has been off the coast of Lebanon and Syria for the past 24 hours, maritime monitors said on Monday.

The vessel has been zigzagging in the eastern Mediterranean since it was released from the British overseas territory of Gibraltar on August 18, after being held for six weeks on suspicion it was delivering oil for Tehran's ally Damascus.

"The Adrian Darya 1 has stopped just 45 nautical miles (83km/52 miles) west of Tripoli, Lebanon, in international waters," TankerTrackers, a maritime traffic monitoring website said.

MarineTraffic said the vessel previously known as Grace 1 was since Sunday off the coast of Syria, where US Secretary of State Mike Pompeo on Friday said it would unload.

Pompeo said he had "reliable information" it was transporting oil to Syria in defiance of wide-ranging sanctions on the regime of Syrian President Bashar al-Assad.

The US Department of Treasury said the vessel was "blocked property" under an anti-terrorist order, and "anyone providing support to the Adrian Darya 1 risks being sanctioned".

The declared and suspected destination of the Adrian Darya has changed multiple times since it was released from Gibraltar.

Experts on Monday said the most likely scenario was for a ship-to-ship transfer of some of the tanker's cargo of 2.1 million barrels before it headed back to Iran.

Samir Madani, co-founder of TankerTrackers, said he believes the Adrian Darya will have to transfer "as much as necessary" of its cargo to one of three Iran-flagged in order to return to Iran via the Suez Canal.

He said if the cargo ends up on one of these tankers, "then the oil will most likely end up going back home to Iran".

But it could also be offloaded on the Sandro, a vessel which Madani says is partially owned by Samer Foz, a US-sanctioned Syrian businessman who is among the country's most prominent industrialists.

"If the oil ends up aboard the Sandro... then that will most likely end up going to Syria" the tanker expert told AFP.

Iran said last week it had "sold the oil" aboard the tanker and that the owner will decide the destination.

It did not identify the buyer or say whether the oil had been sold before or after the tanker's detention in the Strait of Gibraltar.

It also said it could not name the actual destination due to "economic terrorism" by the United States and its sanctions on Iran's oil sales.

Tensions between arch-enemies Iran and the US have soared ever since Washington stepped up its campaign of "maximum pressure" against Tehran and reimposed sanctions after leaving the landmark 2015 nuclear deal last year.