"Oil from the #AdrianDarya1 has been offloaded in Syria, proving that Iran lied to the UK and Gibraltar ... EU members should condemn this action, uphold the rule of law, and hold Iran accountable," Pompeo wrote in a post on Twitter.
The United Kingdom seized the tanker in July off the coast of Gibraltar in July.
After authorities received formal written assurances from Tehran that the ship would not discharge 2.1 million barrels of oil from Syria, the vessel was released.
But in September, Britain's foreign minister said the tanker had sold its crude oil to the Assad regime in Syria, breaking those assurances.