The U.S. Treasury Department on Thursday warned that whoever helps fuel Iranian vessels blacklisted by Washington runs the risk of being designated as well.

The Treasury Department blacklisted the Adrian Darya, a tanker at the center of a confrontation between Washington and Tehran, on August 30.

Washington has warned that it would regard any assistance given to the ship as support for a terrorist group, namely, Iran's Islamic Revolutionary Guard Corps.

The ship, formerly called Grace 1, was detained by Britain off Gibraltar in July due to British suspicion it was carrying Iranian oil to Syria in violation of European Union sanctions.

In an update to its frequently asked questions on Iran sanctions on its website, Treasury's Office of Foreign Assets Control (OFAC), said "the bunkering by non-U.S. persons of an Iranian vessel that has been identified as blocked property of an Iranian person... and the making of related payments for these bunkering services – risk being designated themselves."

OFAC said anyone helping to fuel an Iranian vessel that was transporting agricultural commodities, food, or medicine to Iran would not be exposed to sanctions, unless the transactions involved people already blacklisted by Washington.

The move outlines how the Trump administration is applying so-called secondary sanctions, or sanctions on entities in third party countries, in regards to the Iranian tanker and any other Iranian vessels it blacklists.

The Adrian Darya was released in mid-August after Iran told Britain its cargo was not headed to Syria. But the ship has been cruising the Mediterranean since its release and appeared to have turned off its transponder this week.

In reports confirmed by the U.S. State Department on Wednesday, a top U.S. envoy for Iran has offered several million dollars to the captain of Adrian Darya 1 to steer it to a port in a country that would detain the ship on the U.S.' behalf. He also threatened to make life harder for the captain if he refused to cooperate, the Financial Times reported.

U.S. Defense Secretary Mark Esper said on Thursday he currently had no plan to seize the vessel.

(With input from Reuters)