Iran's Islamic Revolutionary Guard Corps (IRGC) said it captured the vessel, the Stena Impero, after it violated "international regulations," according to the country's state-run Press TV.
A second ship, the Liberian-flagged MV Mesdar, was briefly held by Iran but then released, United States officials and the company that owns the ship said. Iran said the vessel was not seized, just "shortly stopped and briefed by Iranian authorities," Press TV reported citing unnamed Iranian military sources.
"These seizures are unacceptable. It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region," British Foreign Secretary Jeremy Hunt said Friday.
"We are absolutely clear that if this situation is not resolved quickly, there will be serious consequences," said Hunt. "We're not looking at military options, we're looking at a diplomatic way to resolve this situation. But we are very clear that it must be resolved."
Speaking to reporters Friday, US President Donald Trump said the US does not have many tankers in the region but that the country does have a robust military presence there.
"They're (Iran) doing this either in response to the seizing of Grace 1 by Royal Marines and its holding in Gibraltar ... or they may be doing it to widen the tension in the Gulf now because they want to bring this conflict and this state of affairs, which is damaging to Iran, to a head," said British lawmaker Bob Seely, a member of the UK Foreign Affairs Committee.
An increase in tensions in the Strait of Hormuz could have dire economic and security consequences. Around 24% of global oil production passes through the narrow passage, and it's the only way to ship oil out of the Persian Gulf. The US Energy Information Administration calls the Strait of Hormuz one of the "world's most important strategic chokepoints by volume of oil transit."
Richard Meade, the managing editor of the influential shipping industry publication Lloyds List, said the Stena Impero's seizure is "probably the highest level security threat that we have seen in the region since the late 80s."
No Britons on board
Though the Stena Impero is registered in the UK, there were no Britons on board when it was seized. The 23-man crew was made up of Indian, Russian, Latvian and Filipino nationals, according to a statement from the ship's owner -- Sweden-based Stena Bulk -- and operator, Scotland-based Northern Marine Management.
The statement from the two companies said their ship was first approached at "by unidentified small crafts and a helicopter" in the Strait of Hormuz in international waters at about 4 p.m. local time (12 p.m. ET).
More than six hours later, the ship was "no longer under the control of its crew" and "uncontactable."
Iran's Fars news agency reported that Iranian authorities accused the Stena Impero of "disregarding the established procedures that require all entries be made through the northern pass." The tanker was then escorted to Iranian coastal waters for further legal procedures and investigations, according to the agency.
The Stena Impero's seizure marks the latest in series of accelerating maritime incidents in the Gulf region between Iranian, UK and US military forces.
On Thursday, the US Navy destroyed an Iranian drone using electronic jamming equipment in the Strait of Hormuz, a US defense official told CNN. The crew of the USS Boxer took defensive action against the Iranian unmanned aerial vehicle after it came close to the US naval ship, the official said.
Following the seizure of the ship Friday, the UK convened a COBRA meeting -- an emergency response committee that meets when there's a domestic or international crisis. The UK has warned ships connected to the country's shipping industry to "stay out of the area for an interim period."
As of Friday evening, the US military was monitoring the transit of a US commercial cargo ship through the Strait of Hormuz, initially using armed aircraft overhead, according to a US defense official with direct knowledge of the situation.
Continuous monitoring of the transit, which typically takes six to eight hours, is being done mainly by unarmed surveillance aircraft overhead that can call in other forces if needed.