The US says it will aggressively enforce its sanctions to prevent the private sector from assisting an Iranian oil tanker.
The supertanker Grace 1, which had its name changed to Adrian Darya-1, set sail towards Greece after its six-week seizure in Gibraltar over concerns that it was taking oil to Syria in breach of EU sanctions.
The US has warned any nation helping the tanker risks facing US sanctions. On Thursday, it repeated calls for its seizure.
"The shipping sector is on notice that we will aggressively enforce US sanctions," a State Department official said.
Ship tracking data has shown the tanker is heading towards Greece, although Greek Prime Minister Kyriakos Mitsotakis said this was not the case.
On Tuesday US Secretary of State Mike Pompeo warned that America would take every action it could to stop the Grace 1 delivering oil to Syria.
"We have made clear that anyone who touches it, anyone who supports it, anyone who allows a ship to dock is at risk of receiving sanctions from the US," Mr Pompeo said.
"If that ship again heads to Syria we will take every action we can, consistent with those sanctions, to prevent that."
Greece said on Wednesday it would not endanger its relations with the US by aiding it.
The vessel is too big to enter any Greek port but could still enter Greek waters or anchor offshore.
The affair is threatening to pull Greece into a growing confrontation between Iran and America after US President Donald Trump pulled Washington out of Tehran's nuclear deal with world powers more than a year ago.
Mr Trump then reimposed strict sanctions on Iran, largely halting the sale of Iran's crude oil internationally and sharply depreciating its currency.
The supertanker left Gibraltar on Sunday despite last-minute attempts by the US to seize it.
Gibraltan authorities said they had received assurances from Iran that the tanker would not unload its cargo in Syria.