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 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.
On 19 July, British-flagged and Swedish-owned oil tanker Stena Impero was seized by Iran’s Revolutionary Guard in the Gulf.
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.
Tensions between Iran and the West can be traced to the resurgence of another crisis – that over Iran’s nuclear programme.
Last year, Washington withdrew from a 2015 deal to limit the Iran’s nuclear activities, amid suspicion that Tehran was still trying to develop nuclear weapons. Something Iran has always denied.
Since then, US-Iran tensions have grown after Washington imposed – and latterly tightened – its sanctions against the country.
The UK and other European countries have said they remain committed to the deal.