An Iranian oil tanker held for six weeks after being impounded left Gibraltar on Sunday, days after the authorities there rejected a request from American officials that it turn the vessel over to them.
A marine traffic monitoring site showed the tanker, the Grace 1, leaving Gibraltar’s waters. Iranian and Gibraltar news organizations confirmed that it had set sail.
The ship’s departure raised hopes that Iran, in turn, would relinquish a British tanker it seized in what appeared to be retaliation.
On Thursday, Gibraltar ordered the release of the Grace 1 despite a last-minute request from the United States hours earlier that it be allowed to seize the ship.
The following day, the Justice Department unsealed a warrant issued by a federal court in Washington to seize the tanker, the oil it contains and nearly $1 million based on statutory violations.
“A network of front companies allegedly laundered millions of dollars in support of such shipments,” Jessie Liu, the United States attorney for the District of Columbia, said in a news release.
The Justice Department said that “multiple parties affiliated with” the Islamic Revolutionary Guard Corps of Iran, which the United States has designated a foreign terrorist organization, were thought to be involved.
On Friday, Gibraltar’s chief minister, Fabian Picardo, said that, because of the American intervention, the case “could go back to the court, absolutely.”
But on Sunday, the government of Gibraltar rejected the American request. It said in a statement that the warrant relied on broad United States sanctions against Iran that were not applicable in the European Union.
Iran’s naval commander said Sunday that his country was ready to dispatch its naval fleet to escort the oil tanker, renamed Adrian Darya-1.
“The era of hit and run is over,” Rear Adm. Hossein Khanzadi was quoted as saying by the Mehr news agency. “If top authorities ask the navy, we are ready to escort our tanker Adrian.”
The decision to release the ship was seen as a sign of easing of tensions between officials in Gibraltar, a semiautonomous British territory, Tehran and London. A confrontation between Iran and the West has escalated in recent weeks, particularly with the United States.
The Grace 1 was seized on July 4 by British marines and Gibraltar port officials, who asserted that the tanker was carrying oil to Syria in violation of a European Union embargo.
Iran soon detained a British-flagged tanker, the Stena Impero in the Strait of Hormuz, the narrow entryway to the Persian Gulf that is a conduit for around 20 percent of the world’s crude oil.
In announcing the decision Thursday to release the vessel, the government of Gibraltar said that it had information that the Grace 1 was headed to Syria when it was initially detained, but that it now had assurances the tanker would not go to there. Three crew members who had been detained were also released.
Some speculated that the release could be part of a ship swap, but none of the involved parties confirmed that information. But the Iranian authorities said on Friday that they had made no promises.
“Iran has made no commitment for the release of the Grace 1 tanker,” said Abbas Mousavi, a spokesman for the Iranian Foreign Ministry, according to a report from the semiofficial Tasnim news agency.
Mr. Mousavi also said, “We announced from the early hours of the oil tanker’s detention that Syria was not the oil tanker’s destination and remained so to the end.” Even if the oil had been headed to Syria, he said, “it would still have nothing to do with anyone.”
The Gibraltar government rejected the claims, saying Iran had indeed made commitments to not transport the oil to Syria. “The evidence is clear, and the facts speak louder than the self-serving political statements we are hearing today,” the government said in a statement.
It was unclear if the United States intended to seize the vessel if it left Gibraltar.
But any attempt to intercept the tanker in international waters would most likely be in violation of international rules.
The United Nations Convention on the Law of the Sea, which provides a framework for engagement at sea, maintains that military ships do not have the authority to detain or board foreign-flagged vessels in international waters unless the ship is suspected of serious offenses like piracy or participating in the slave trade.
The United States has announced additional punitive measures for the crew members of any vessel that transports oil from Iran to Syria.
In a statement released late Thursday, the State Department said it would revoke United States visas for the crew members operating Grace 1. The State Department said that it had found that the vessel was helping the Islamic Revolutionary Guards Corps by transporting Iranian oil.
“This could result in serious consequences for any individuals associated with the Grace I,” the statement said.
Secretary of State Mike Pompeo posted the statement on Twitter with a warning: “A message to all mariners — if you crew an IRGC or other FTO-affiliated ship, you jeopardize future entry to the U.S.”