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.