The U.S. Treasury Department on Friday blacklisted the Iranian oil tanker Adrian Darya, which is at the center of a confrontation between Washington and Tehran, and sanctioned its captain.

"Vessels like the Adrian Darya 1 enable the (Islamic Revolutionary Guard Corps-Quds Force) to ship and transfer large volumes of oil, which they attempt to mask and sell illicitly to fund the regime's malign activities and propagate terrorism," Treasury Under Secretary Sigal Mandelker said in a statement.

Previously known as Grace 1, the ship has been bouncing around the Mediterranean after being held for six weeks by Gibraltar on suspicion its cargo was bound for Syria.

Despite Washington's efforts to keep it detained, it was released by the British territory and its every move is being followed with intense speculation.

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

The ship was seized by the Gibraltar police and British special forces on July 4 and held on suspicion of shipping oil to Syria in breach of European Union sanctions.

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

In July, Iran's Islamic Revolutionary Guard Corps impounded a British-flagged tanker in strategic Gulf waters.

Britain called it a tit-for-tat move but Tehran denied any connection. A court in the British territory ordered the tanker's release on August 15, despite a last-minute legal bid by the United States to have it detained.

Tensions between Iran and the U.S. have soared ever since Washington stepped up its campaign of "maximum pressure" against Tehran and re-imposed sanctions after leaving the landmark 2015 nuclear deal last year.

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A year of U.S. sanctions on Iran

Syria, which has ports on the Mediterranean, is also under a raft of U.S. and European sanctions over its eight-year conflict. 

Russia, which together with Iran, is Damascus' key ally in the conflict, announced Friday that a ceasefire would come into force in the northwestern region of Idlib.

(With input from Reuters, AFP)