Gibraltar, a British overseas territory on the southern tip of Spain, said the US was seeking to seize the ship for reasons "intrinsically linked" to US sanctions against Iran, the Gibraltar Chronicle reported.
There were no such sanctions in Gibraltar, Britain or the rest of the EU, it said.
"The EU sanctions regime on Iran is fundamentally different to that of the US," the report cited the Gibraltar government as saying.
The Grace 1, which has since been renamed as the Adrian Darya 1, was captured on July 4 off the coast of Gibraltar, with London alleging the vessel was carrying Iranian oil to Syria in violation of EU sanctions.
The seizure escalated tensions between London and Tehran, with Iran later seizing the British-flagged Stena Impero in the Strait of Hormuz, a strategically important shipping route in the Gulf region.
The US warrant alleged "a scheme to unlawfully access the US financial system to support illicit shipments to Syria from Iran by the Islamic Revolutionary Guard Corps (IRGC), a designated foreign terrorist organization," the Justice Department said.
Iran said on Sunday that it expects the tanker to leave Gibraltar soon.
"Two expert teams are on their way to Gibraltar to get the ship ready ... we assume that the tanker can leave tonight," Iran's ambassador to London, Hamid Baeidinejad, wrote in a tweet.
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