A US court has issued a warrant for the seizure of an Iranian tanker that British Royal Marines had seized last month in Gibraltar, a court document showed on Friday.

The oil tanker Grace 1, the more than 2 million barrels of oil it carries and $995,000 are subject to forfeiture based on a complaint by the US government, US Attorney for the District of Columbia Jessie Liu said in a news release.

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The tanker was seized by British Royal Marines at the western mouth of the Mediterranean on July 4 on suspicion of violating European Union sanctions by taking oil to Syria, a close ally of Iran.

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.

Gibraltar lifted the tanker's detention order on Thursday but the vessel's fate was further complicated by a last-ditch US legal appeal to hold it.

The tanker shifted its position on Friday, but its anchor was still down off Gibraltar and it was unclear if it was ready to set sail soon.

The warrant for the seizure of the tanker, which carries 2.1 million barrels of oil, was issued by the US District Court for the District of Columbia and addressed to "the United States Marshal's Service and/or any other duly authorized law enforcement officer."