An Iranian oil tanker blacklisted by the U.S. Treasury is steaming slowly near the coast of Syria. The ship-tracking website MarineTraffic.com reveals the Adrian Darya 1 slowed to a near-stop on Sunday some 50 nautical miles off Syria.

The vessel still does not list a destination for its 2.1 million barrels of oil, worth some $130 million.

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 E.U. sanctions.

It was released earlier this month after Iran gave assurances it would not discharge its cargo in Syria and the U.S. has since tried, unsuccessfully, to have the ship detained again.

In a statement, the U.S. Treasury claimed 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 the U.S. has designated a terrorist organisation.

It insisted ships like Adrian Darya 1 were being used to sell oil illicitly to fund Iran’s “malign activities and propagate terrorism”.

U.S. 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.

It was “a big mistake”, he added, to trust any assurances about the ship from Iranian Foreign Minister Javad Zarif.

FM @JZarif guaranteed to the UK that the IRGC oil tanker #Grace1 / #AdrianDarya1 would not head to Syria. We have reliable information that the tanker is underway and headed to Tartus, Syria. I hope it changes course. It was a big mistake to trust Zarif. pic.twitter.com/ZJ06MWjvCO — Secretary Pompeo (@SecPompeo) August 30, 2019

After the Adrian Darya 1 left Gibraltar last week, it listed the Greek port Kalamata as its final destination after Pompeo threatened sanctions on any country that helped the tanker. Greece later said it would not “facilitate” its course to Syria.

Meanwhile, Iran’s deputy foreign minister and economists are to travel to Paris on Monday to speak with French officials.

AP contributed to this report.