Britain summoned on Tuesday the Iranian ambassador to condemn a clear breach of the assurances it was given over the oil cargo of the tanker Adrian Darya 1, which had previously been detained for breaching European Union sanctions.
In mid-August, a Gibraltar court ordered the vessel's release against USA objections, after authorities said they had received assurances from Iran that the ship would not proceed to Syria.
The ship, seized in July with the help of UK Royal Marines, was released after Iran gave written assurances that it was not bound for Syria.
United States of America since November of 2018 was given the eight countries and territories - China, Greece, India, Italy, Japan, South Korea, Taiwan and Turkey - six months for continued imports of Iranian oil, but on may 2 the effect of these exceptions ended.
"It is now clear that Iran has breached these assurances and that the oil has been transferred to Syria and (President Bashar al-Assad's) murderous regime", the Foreign Office said.
"The world will be better placed to judge Iran's position on other, more transcendental matters based on actions on this issue on which they have been under the microscope of global scrutiny", it said in a statement.
Iran also seized a British-flagged tanker in the strategic Strait of Hormuz in a retaliatory action.
The UK also said if were to backtrack on their assurances they would be in "violation of global norms and a morally bankrupt course of action".
France has proposed offering Iran $15 billion in credit lines if Tehran comes back and complies with the 2015 nuclear deal.
Mandelker said that Washington "will continue to impose sanctions" against any who would buy Iranian oil or to conduct any business with the IRGC.
The ship was photographed off the coast of on Friday and is believed to have been carrying 2.1 million barrels of Iranian crude oil.