The captain of an Iranian supertanker at the centre of a diplomatic standoff no longer wants to keep command of the ship, which is in need of repairs that could impede its immediate departure from Gibraltar, the sailor's lawyer said Friday.

Any delay in the Grace 1's departure could provide a window for the United States to mount further legal action in Gibraltar, seeking to seize the tanker amid a growing confrontation with Tehran.

U.S. authorities announced in Washington late Friday afternoon that they had obtained a warrant to seize the tanker, though Gibraltar court officials said they had not received any claim by the end of the business day in the British overseas territory. It wasn't immediately clear if the U.S. had forwarded the warrant.

The tanker — and its 2.1 million barrels of Iranian light crude oil — seemed to perform mild manoeuvres Friday but largely remained still in waters off Gibraltar, a day after authorities ended its detention for allegedly breaching European Union sanctions on Syria.

The release Thursday came over the objections of the United States. The head of the Gibraltar government said Iran had promised him not to deliver the fuel to a sanctioned refinery in Syrian territory. An Iranian official later disputed that those assurances had been delivered.

Tensions between Iran and the U.S. have escalated since President Donald Trump last year unilaterally pulled the U.S. out of the 2015 nuclear deal signed by Iran and other world powers.

The decision reimposed sanctions on Iran, stopping billions of dollars in business deals, largely halting the sale of Iran's crude oil internationally and sharply depreciating Iran's currency, the rial. More recently, the Persian Gulf has seen attacks on oil tankers and other high-stakes confrontations.

Stena Impero, a British-flagged vessel owned by Stena Bulk, is seen at Bandar Abbas port in Iran on July 21. Iran seized the vessel in retaliation for the Grace I being detained. (Mizan News Agency/WANA Handout/Reuters)

In early July, Tehran seized the British-flagged oil tanker Stena Impero in apparent retaliation for the detention of the Grace 1. Analysts had said the release of the Grace 1 by Gibraltar could see the Stena Impero go free.

But that prospect remained up in the air Friday, with a lawyer representing three Grace 1 crew members who were released from detention Thursday, casting doubt on the vessel's immediate departure.

Richard Wilkinson told The Associated Press that the Indian national who commanded the oil tanker until it was detained in early July had asked his Iranian employers to replace him.

"He doesn't want to stay in command of the ship, he wants to go home, because he wasn't happy to go back and pick up the broken pieces," the attorney said. "But he's a professional skipper and needs to wait for a new crew to do a proper handover."

The lawyer said the tanker had been due for repairs in Gibraltar even before it was seized, and its detention impeded the replacement of parts, making the tanker unfit for an immediate long voyage.

Adding to the uncertainty, the next possible destination of the cargo became a point of contention as Iranian and Gibraltar authorities showed disagreement over the terms that led to the ship's release.

'Syria was not its destination'

Iranian Foreign Ministry spokesperson Abbas Mousavi said Friday that his country had made no commitments to gain the release of its tanker, while the chief minister of Gibraltar insisted written assurances were received.

The captors of the vessel "raised the issue of commitment in a bid to make up for their humiliation caused by this illegal act and piracy," Mousavi said, according to the semi-official Tasnim news agency.

"We announced that Syria was not its destination and we have upheld the same … and reiterated that it was nobody's business even if it was Syria," he added.

In response, the Gibraltar government issued a statement saying that "the evidence is clear and the facts speak louder than the self-serving political statements we are hearing today."

Authorities in the territory at the mouth of the Mediterranean Sea didn't reveal if the vessel was expected to leave soon. Gibraltar's Chief Minister Fabian Picardo said it was strictly a matter of the ship's agents and owners.

Authorities in Gibraltar had said Thursday that the U.S. Justice Department had asked to seize the ship and its cargo, but Picardo said such a request should be made directly to the territory's judiciary and not the government.

Gibraltar's Chief Minister Fabian Picardo did not reveal when the tanker is expected to leave the area, saying it was up to the ship's agents and owners. (Daniel Leal-Olivas/AFP/Getty Images)

The U.S. government said Friday it had obtained a warrant to seize the tanker, citing violations of U.S. sanctions, as well as money laundering and terrorism statutes. The U.S. is seeking to take control of the tanker as well as all of the petroleum aboard it and $995,000 US.

In court documents unsealed in Washington, the U.S. alleges a complex scheme to unlawfully access the U.S. financial system in order to support shipments to Syria by Iran's Islamic Revolutionary Guard Corps, which has been designated as a foreign terrorist organization.

The complaint alleges that people associated with the ship provided fraudulent shipping documents, had obtained oil and departed from an Iraqi shipping port. The U.S. says the documents were forged to mask the fact that the ship had actually departed from Iran.

The court papers allege the Grace 1 is owned and managed by separate companies that appear to be operating on behalf of other parties. But, the U.S. government alleges the tanker is ultimately controlled by the Revolutionary Guard Corps.

A senior administration official said the Justice Department's complaint provides additional details associating the ship with the Revolutionary Guard Corps and its support for terrorism.

The official said the Iranian tanker constitutes some $130 million worth of oil that was destined for a designated terror organization to conduct more terrorism, and that is a compelling case for seizure.

The official said that even if Gibraltar authorities don't think the filing warrants further detention of the vessel, no matter where it goes, "our actions are going to follow it like a cloud." The official said that includes crew members, owners and "anyone who touches this ship in any way."

The Justice Department's action comes after the U.S. State Department warned that anyone assisting the Grace 1 may not be eligible for visas or for admission into the United States.