Late Friday, a management company for the Liberian tanker, Mesdar announced in a statement that "the armed guards have left and the vessel is free to continue the voyage. All crew are safe and well."
Foreign Secretary Jeremy Hunt said in a statement he was "extremely concerned by the seizure of two naval vessels by Iranian authorities."
Iran's State TV said on Saturday that the British tanker was seized after it collided with a fishing vessel and ignored calls from the smaller craft.
Late Friday evening, following meetings in Washington, D.C., with more than 60 foreign ambassadors, U.S. Central Command (CENTCOM) announced a U.S.-led maritime security initiative dubbed Operation Sentinel "to increase surveillance of and security in key waterways in the Middle East to ensure freedom of navigation in light of recent events in the Arabian Gulf region."
The initiative, according to CENTCOM, "will nations to provide escort to their flagged vessels while taking advantage of the cooperation of participating nations for coordination and enhanced maritime domain awareness and surveillance." The statement does not specify the nations that have joined the coalition.
The British tanker, the Stena Impero, was crossing through the strait when it abruptly changed course and headed north towards Iran's Qeshm island, ship tracking sites showed.
That tanker's owner and management companies issued a statement that the ship was "approached by unidentified small crafts and a helicopter" while in international waters. "We are presently unable to contact the vessel which is now heading north towards Iran," the companies, Stena Bulk and Northern Marine Management, said in the statement.
The Islamic Revolutionary Guards Corps (IRGC) said that its forces had seized the Stena Impero "due to non-compliance with international maritime law and regulations," according to a statement carried by Iranian state news agencies. The tanker had been guided to shore and was already in port, the statement said.
The U.K. government has convened an emergency session of COBRA, its crisis committee, a senior U.K. official told ABC News, speaking on condition of anonymity because the person was not authorized to comment publicly. Hunt confirmed that he was attending the meeting to review "what we know and what we can do to swiftly secure the release of the two vessels."
There were 23 crew aboard the Stena Impero, according to the statement from its owner, and currently there are no reports that any have suffered injury. Hunt said both vessels had crews comprised of a "range of nationalities," but noted there were no British citizens on board either ship.
The seizure comes less than a month after the U.K. seized an Iranian oil tanker off the coast of Gibraltar, accusing it of violating European Union sanctions by trying to ship oil to Syria.
Iran has repeatedly vowed retaliation since then and last week a British warship intervened to chase away Iranian small boats that approached another British tanker in the Strait of Hormuz.
Tensions have spiked in the strait again in the past few days after Iran said it had seized another Panamanian flagged tanker on Thursday accused of oil smuggling and the U.S. said it had destroyed an Iranian drone that approached ones of its warships. Iran has denied any of its drones were destroyed.
President Donald Trump said Thursday that the U.S. would be speaking to British officials about the situation and noted that the U.S. doesn’t have many tankers passing through the area.
"We heard that,” Trump told reporters as he departed the White House en route to New Jersey. "The United States has very few tankers going in because we're using our own energy now. We've made a lot of progress over the last two and a half years. So we don't have very many tankers going in, but we have a lot of ships there that are warships, and we'll talk to the U.K."
The government did say a U.S.-flagged ship, the Maersk Chicago, was traveling through the Strait of Hormuz late Friday and was being monitored.
"We have patrol aircraft operating in international airspace monitoring the situation within the Strait of Hormuz," said Lt. Col. Earl Brown, CENTCOM Chief of Media Operations. "U.S. Naval Forces Central Command has been in contact with U.S. ships operating in the area to ensure their safety."
The president noted multiple times that the U.S. and U.K. don’t have a “written agreement,” but that the two countries were strong allies and that "they’ll have a new prime minister soon, which is a good thing."
Later, he said that Iran was “trouble, nothing but trouble.”
The White House National Security Council spokesperson Garrett Marquis said the council is aware of the reports of the tanker's seizure.
"We are aware of reports that Iranian boats seized a British oil tanker," he said. "This is the second time in just over a week the U.K. has been the target of escalatory violence by the Iranian regime. The U.S. will continue to work with our allies and partners to defend our security and interests against Iran’s malign behavior."
ABC News' Elizabeth McLaughlin and Cindy Smith contributed to this report.