The US Treasury has blacklisted an Iranian tanker it claims is being used to ship oil to Syria.
The Adrian Darya 1, previously known as Grace 1, was detained by British authorities in Gibraltar in July after it was suspected of moving oil to Syria in breach of EU sanctions.
It was released earlier this month after Iran gave assurances that it would not discharge its cargo in Syria.
The US has since tried, unsuccessfully, to have the ship detained again.
Its initial seizure sparked a diplomatic crisis between the UK and Iran, which saw Iran seize a British-flagged and Swedish-owned oil tanker, the Stena Impero, in the Gulf, reports the BBC.
In a statement, the US Treasury said the Adrian Darya 1 was being used to transport 2.1 million barrels of Iranian crude oil for the benefit of Iran's Revolutionary Guard - a branch of the country's armed forces that the US has designated a terrorist organisation.
It insisted that ships like Adrian Darya 1 were being used to sell oil illicitly to fund Iran's "malign activities and propagate terrorism".
US Secretary of State Mike Pompeo added in a Tweet that America had "reliable information" the vessel was on course to Tartus, a major Syrian port city which is also home to a Russian naval base.
He added that it was "a big mistake" to trust any assurances about the ship from Iranian Foreign Minister Javad Zarif.
In earlier talks, Iran said that oil aboard the ship had already been sold, but it did not revealed the identity of the buyer.
After the Adrian Darya 1 left Gibraltar last week, it listed the Greek port Kalamata as its final destination.
Mr Pompeo threatened sanctions on any country that helped the tanker, and Greece later said it would not "facilitate" its course to Syria.
According to publicly available tracking data, the Adrian Darya 1 has changed its final destination to Iskenderun in eastern Turkey, abouit 25 miles (40km) from the Syrian border.
But Turkish Foreign Minister Mevlut Cavusoglu said on Friday that the tanker had changed course several times and insisted that it was heading for Lebanese waters instead.
