Gibraltar security forces aided by British Royal Marines intercepted the Grace 1 Iranian supertanker off the coast of the British overseas territory on July 4.
Adrian Darya 1, formerly Grace 1 was seized by British Royal Marines on July 4 on suspicions of carrying oil for Syria in violation of European Union sanctions.
"This sale of oil to Assad's brutal regime is part of a pattern of behavior by the government of Iran created to disrupt regional security", he said.
"We will continue to put pressure on Iran and as [U.S.] President [Donald Trump] said there will be no waivers of any kind for Iran's oil", U.S. Treasury Under Secretary for Terrorism and Financial Intelligence Sigal Mandelker told reporters on Sunday.
The Foreign Office said it was clear the ship's cargo had been transferred to the Assad regime in an "unacceptable violation" of worldwide norms.
The UK also said if were to backtrack on their assurances they would be in "violation of worldwide norms and a morally bankrupt course of action".
"Iran has shown complete disregard for its own assurances over Adrian Darya 1", said Dominic Raab, the Foreign Secretary.
"Iran's actions represent an unacceptable violation of worldwide norms", the statement said.
The US then issued a warrant for the ship to be seized, naming it among a network of tankers which it said were being used by Iran's Revolutionary Guards to transport oil to Syria.
The US has said they would impose sanctions on any buyer of the oil.
The US attempted to block the departure of the ship from Gibraltar, and US administration officials then tried to contact the captain of the ship to offer him a substantial sum in return for not selling the oil to Syria but instead taking the ship to a port at which the US could seize the 2.1m tonnes of oil.
France has proposed offering Iran $15 billion in credit lines if Tehran comes back and complies with the 2015 nuclear deal.