The Marshall Islands-flagged tanker, the Talara, was boarded by Islamic Revolutionary Guard Corps (IRGC) forces who arrived by helicopter, US Central Command (CENTCOM) said in a new assessment on Sunday, two days after the hijacking.
"Iran's use of military forces to conduct an armed boarding and seizure of a commercial vessel in international waters constitutes a blatant violation of international law, undermining freedom of navigation and the free flow of commerce," CENTCOM's statement read.
CENTCOM called on Iran to make a legal argument for its actions to the international community.
"US forces will continue to remain vigilant and work alongside our partners and allies to promote regional peace and stability."
Iran takes responsibility for seizing oil tanker
The incident, which took place on Friday morning, was originally noticed by a UK maritime security company and later confirmed by US officials.
The tanker had been sailing off the coast of the United Arab Emirates and was carrying a cargo of high-sulfur gasoil through the Indian Ocean en route to Singapore from Sharjah in the UAE.
Iran confirmed the seizure on Saturday in a statement from the IRGC, read on Iranian state-run television, stating that "the tanker was in violation of carrying unauthorized cargo."