The US Department of Justice has put in a request to detain supertanker Grace 1, which will be considered by Gibraltar’s Attorney General.
The vessel, carrying Iranian oil, was seized by Royal Marines on 4 July – triggering a standoff with Tehran.
It was set to be released within hours, after the attorney general indicated he would make no further order for its detention.
But after the US application, the matter has been adjourned until 15:00 BST.
Gibraltar said the US application is based on a “number of allegations which are now being considered”.
A government spokesman also confirmed that police proceedings against all four members of the crew, including the captain – an Indian national – had ended.
The crew’s legal team told the BBC that police had said this was due to actions by the Iranian government, and that it was no longer in the public interest to proceed.
The tanker was first seized after the government of Gibraltar suggested it was heading for Syria, in breach of EU sanctions.
About 30 marines were flown from the UK to Gibraltar to help police detain the tanker and its cargo, at the request of the Gibraltar government.
The initial seizure of the tanker sparked a diplomatic crisis between the UK and Iran, which has escalated over recent weeks.