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 to 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 departed.
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 about 20 percent of the world’s crude oil.
The decision to release the Iranian ship was seen as a sign of easing of tensions between Gibraltar, a semiautonomous British territory, London and Tehran. A confrontation between Iran and the West, particularly with the United States, has escalated in recent weeks.
The ship’s departure also raised expectations that Iran, in turn, would relinquish the Stena Impero.
Gibraltar ordered the release of the Grace 1 on Thursday despite a last-minute request from the United States 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 Guards Corps of Iran, which the United States has designated a foreign terrorist organization, were believed involved.
On Friday, Gibraltar’s chief minister, Fabian Picardo, said that because of the American intervention, the case would go back to court.
But on Sunday, the Gibraltar government rejected the American request. It said that the warrant had 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 Iran’s Mehr news agency. “If top authorities ask the navy, we are ready to escort our tanker Adrian.”
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 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. 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 disputed Iran’s 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.
It was unclear whether the United States intended to seize the vessel now that it has left Gibraltar. But any attempt to intercept the tanker in international waters would most likely be considered illegal.
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.
The State Department said Thursday 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 State Department 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.”