The MT Tour tanker carrying Syrian crude oil dropped anchor in the Strait of Hormuz near the Iranian port of Bandar Abbas over the weekend, AIS Live ship tracking data on Reuters showed on Sunday.
Syria, a small exporter at 109,000 barrels per day in 2010, used to sell most of its crude to Europe until European Union sanctions on oil trade with President Bashar Al-Assad's government stopped those sales last year.
The MT Tour is owned by ISIM Tour Limited, a shipper identified by the United States as an Iranian government front company set up to evade sanctions. It loaded Syrian light crude in the Mediterranean in late March before sailing through the Suez Canal and then heading east.
Iran sent the tanker to pick up the crude, estimated to be worth about $84 million to its sanctions-hit Syrian ally, and a source told Reuters last month the cargo was destined for China.
It is not clear why the MT Tour has now dropped anchor off Iran, which as one of the world's biggest crude producers has no reason to import more.
Tehran is finding it difficult enough to find buyers for its own oil as European and Asian customers cut back imports due to sanctions, forcing it to store millions of barrels of its own crude in oil tankers.
The MT Tour is one of the few Iranian oil tankers whose location is still visible on AIS, while most have apparently switched off their onboard tracking systems.
Of those still visible, a fully loaded Iranian very large crude carrier (VLCC) called the MT Nabi was heading north on Sunday in the Red Sea towards the off-loading port for the Sumed pipeline, which crosses Egypt carrying oil from the Gulf to the Mediterranean.
Fully loaded VLCCs cannot sail through the Suez Canal, but the MT Nabi can send its cargo through Sumed for it to be reloaded at the other end and shipped to Europe or the Americas.
Although many buyers in Europe have already sharply reduced or stopped importing Iranian oil, a European Union ban on Iranian oil imports does not come into force until 1 July.
Another Iranian VLCC called the Delvar is three days from its delivery destination of Dalian, China, while a VLCC called Dena was still showing as off Durban, South Africa, where it has been anchored since early March. Its position in the water indicates the Dena is no longer fully loaded with oil.
Two smaller Iranian oil tankers, the Abadan and Amol, were also anchored near Bandar Abbas on Sunday, AIS Live ship tracking data shows.