Grace 1 tanker was detained by authorities in Gibraltar on July 4 with the help of Royal Marines on the suspicion that it was ferrying over 2 million barrels crude oil to Syria in breach of European sanctions. The move triggered a standoff with Tehran.
The vessel was due to be released on Thursday after Gibraltar's Attorney General indicated there would be no further order for its detention.
But the Gibraltar government later said that the US Department of Justice sought to extend the detention of Grace 1, prompting the Supreme Court in the territory to adjourn a scheduled decision on whether to release the ship until later in the day, the BBC reported.
"The US Department of Justice has applied to seize the Grace 1 on a number of allegations which are now being considered," the government said in a statement, adding that the matter would be reviewed by the court at 4 p.m. (local time).
A spokesman for Gibraltar's government said that police proceedings against all four members of the crew, including the captain - an Indian national - had ended, according to the BBC.
Britain's Foreign Office said in a statement that it was unable to comment on the US request as "the "investigations conducted around the Grace 1 are a matter for the government of Gibraltar".
The seizure of the tanker in July led reprisals by Tehran, including the capture of the British-flagged Stena Impero in the Gulf.
Both incidents fuelled worsening hostilities between Iran and the West that began when the US pulled out of an international agreement curbing Iran's nuclear programme in 2018 and reimposed economic sanctions.
Last week, the UK announced it would join a US-led taskforce to protect merchant ships travelling through the key shipping route in the Strait of Hormuz.
( With inputs from IANS )