Updated at 3:20 a.m. Monday
The United Kingdom is trying to defuse an escalating standoff with Iran just days before Britain's ruling Conservative Party announces the successor to Theresa May, who is resigning.
Some 160,000 members of the Tory Party have until today to return their ballots selecting a new leader. The winner, to be announced on Tuesday, is expected to be Boris Johnson.
Johnson is a mercurial, pro-Brexit former foreign secretary who was also the London mayor and is not known for his diplomacy. If he's installed in office on Wednesday, as expected, he will inherit a full-blown international crisis, which erupted on Friday after Iran's Revolutionary Guard seized a British-flagged commercial oil tanker named the Stena Impero in the Strait of Hormuz.
Britain has warned its ships to stay out of the strait, which has become a focal point for tensions between the United States and Iran. Commercial tankers carry a fifth of the world's crude oil through the strait.
British Defense Secretary Penny Mordaunt called Iran's seizure of the Stena Impero and its 23 crew members "a hostile and illegal act." She insists the tanker, which was headed to a port in Saudi Arabia, was in the territorial waters of Oman when Iran's Revolutionary Guard raided it. Iran claims the Stena Impero had collided with an Iranian fishing vessel and violated maritime safety.
But Foreign Secretary Jeremy Hunt — who is the other candidate for prime minister — told reporters on Saturday that Iran wasn't hiding that the move was likely retaliatory. Hunt made the statement after speaking to his Iranian counterpart, Javad Zarif, by phone.
"It's clear from talking to him, and also statements made by Iran, that they see this as a tit for tat situation following Grace 1 being detained in Gibraltar," Hunt said.
Grace 1 is the Iranian-flagged tanker that Britain's Royal Marines raided on July 4 as it passed through waters off the coast of Gibraltar. The British say they impounded the tanker because they suspected it was heading to Syria in an attempt to circumvent European Union sanctions against the regime of Syrian leader Bashar al-Assad. The British are still holding the ship.
On Twitter, Zarif called the seizure of the tanker "piracy" and said Britain must "cease being an accessory to #Economic Terrorism of the U.S." Zarif insists Iran was upholding maritime law by impounding the Stena Impero.
Maritime security firm Dryad Global released audio on Sunday of the minutes before the tanker's capture. The Stena Impero's fate plays out in two radio calls. In one, an officer in Iran's Revolutionary Guard says, in English: "You are ordered to change your course to 3-6-0 degrees immediately. If you obey you will be safe." In another call, a Royal Navy officer on a British warship, the Montrose, tells the crew of the Stena Impero to stay on course.
"As you are conducting transit passage in a recognized international strait, under international law your passage must not be impaired, obstructed or hampered," the officer says.
The British say the Montrose prevented Iran from interfering with another British-flagged tanker earlier this month. But this time, the Montrose was more than an hour away, according to Mordaunt, the British defense secretary.
In footage released by Iran's Fars News Agency, Iranian speedboats and helicopters surrounded the Stena Impero. Masked marines from Iran's Revolutionary Guard are seen scaling down ropes from the helicopters onto the oil tanker. Soon, the Stena Impero was heading off course to the port of Bandar Abbas in Iran.
The Iranian government says the crew members, who hail from India, Russia, Latvia and the Philippines, are "safe and in good health." Of the 23 crew members, 18 are from India. "We are ascertaining further details on the incident," Indian government spokesman Raveesh Kumar said in a statement. "Our Mission is in touch with the Government of Iran to secure the early release and repatriation of Indian nationals."
The British are demanding the immediate release of the tanker and its crew — so far, to no avail. The U.K. Foreign Office also has summoned Iran's top diplomat in Britain.
Hunt says the tanker's seizure "shows worrying signs Iran may be choosing a dangerous path of illegal and destabilizing behaviour." He said the British government's reaction will be "considered but robust." Britain's Daily Telegraph newspaper reports that the U.K. is expected to announce "diplomatic and economic measures" against Iran on Monday.
U.S. Secretary of State Mike Pompeo, who is traveling in Latin America, said Iran needs to be open to talks.
"The Iranian regime has to make a decision that it wants to behave like a normal nation, and if they do that, we're prepared to negotiate across a broad spectrum of issues," he said.
Iran's seizure of the Stena Impero is also prompting concern across Europe. In a statement, France said it condemns the action by Iran and expresses its "full solidarity with the United Kingdom." Germany released a statement calling the tanker's seizure "unjustifiable" and asking for Iran to immediately release the ship and its crew.
Britain, France and Germany are signatories to the 2015 Iran nuclear deal, which President Trump announced he would withdraw from last year. Under the pact, Iran agreed to curtail its nuclear work in exchange for the easing of sanctions. The U.S. withdrawal from the agreement was followed by new economic sanctions on Iran.
Iran has since breached the terms of the agreement, enriching uranium past the limit set by the deal.
Last week, John Negroponte, former U.S. director of national intelligence and ambassador to the United Nations, told NPR that Iran's breaches of the deal bring it closer to building a nuclear bomb.
"Enriching and stockpiling at levels higher than those agreed in the JCPOA would be a step toward nuclear breakout," Negroponte said, in reference to the nuclear deal. "Iran's newly announced levels appear modest at the moment, but would become more concerning if there were further increases. Such steps would imply a willingness on Iran's part to go all the way to construction of a bomb."
One-fifth of the world's oil supplies travel through the Strait of Hormuz, with tankers carrying crude from the Middle East to countries around the world. The waterway has been a focal point of escalating tensions between the U.S. and Iran.
The U.S. has said it is sending troops and air defense missiles to Saudi Arabia. Late Friday, the United States Central Command said it is working on a "multinational maritime effort" called 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."
NPR's Lauren Frayer contributed reporting.