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”.
Earlier, the legal team for the captain, chief officer and a third crew member of the Grace 1 told the BBC formal police proceedings against them had ended.
They said police told them this was due to undertakings given by the Iranian government and that it was no longer in the public interest to proceed.
The legal team added their clients all want to go home subject to the formal handover of the vessel’s captaincy.
The tanker was first seized due to evidence the tanker 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.