The US Department of Justice has made a last-minute application to block the release of the Iranian oil tanker Grace 1, which was seized by authorities in Gibraltar last month, a spokesman for the Gibraltar attorney general told CNN.

Gibraltar attorney general Joseph Triay revealed the application had been made at a short hearing at the Supreme Court on Thursday morning.

A decision by the court on whether the tanker could be released was expected on Thursday morning, but has now been delayed until later in the day because of the US request, the spokesman added.

The Gibraltar Chronicle reported that a judge in the hearing said the ship would have been freed were it not for the US intervention.

“The US Department of Justice has applied to seize the Grace 1 on a number of allegations which are now being considered. The matter will return to the Supreme Court of Gibraltar at 4:00 p.m. today,” the Gibraltar government said in a statement.

It was not immediately clear what the US Department of Justice allegations were.

A spokesman for Gibraltar's attorney general announced that the captain of the Grace 1 and three officer would be released.

British Royal Marines and Gibraltar port and law enforcement agencies stormed the ship on July 4 under the belief that the vessel carried oil destined for Syria, which would have been in violation of European Union sanctions.

“We have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Baniyas Refinery in Syria,” Spain's acting foreign minister, Josep Borrell, said at the time, adding that the United States had asked the UK to intercept the ship.

Gibraltar is an British overseas territory on the edge of southern Spain. Iran has criticized the seizure as “illegal” and condemned the operation as “piracy,” according to a report at the time from the semi-official Tasnim news agency.

Less than two weeks later, Iran's Islamic Revolutionary Guard Corps (IRGC) said its navy had captured a British-flagged oil tanker, Stena Impero, in the Strait of Hormuz and accused it of “violating international regulations.”

Jeremy Hunt, the UK's then-Foreign Secretary, said in a statement that the incident showed “worrying signs Iran may be choosing a dangerous path of illegal and destabilizing behavior,” and added that the UK's response would be “considered, but robust.” Nearly a month later, the tanker — and its crew — remain detained in Iran.

Prior to the seizures, tensions in the Persian Gulf had been steadily escalating after Iran confirmed it would stop complying with several parts of the 2015 nuclear deal, which the Trump administration withdrew from last year.