Washington had attempted to detain the Grace 1 on the grounds that it had links to Iran’s Islamic Revolutionary Guard Corps (IRGC), which it has designated a terrorist organization.

“The scheme involves multiple parties affiliated with the IRGC and furthered by the deceptive voyages of the Grace 1,” Liu said. “A network of front companies allegedly laundered millions of dollars in support of such shipments.”

“A seizure warrant and a forfeiture complaint are merely allegations. The burden to prove forfeitability in a civil forfeiture proceeding is upon the government,” the news release said.

There was no immediate word from Britain or Gibraltar over whether they would act on the warrant, as Iran said it was sending a new crew to pilot the tanker and its 2.1 million barrels of oil.

“For the release of the Grace 1 oil tanker, Iran has not made any commitments not to send it to Syria,” adding that Iranian officials had previously stated that the destination of the ship was not Syria, but “even if it were… it still has nothing to do with anyone.”

Iranian Foreign Minister Mohammad Javad Zarif tweeted that the US attempt at "piracy" had failed, saying it showed Washington's "contempt for the law."