There will be "serious consequences" if Iran does not release a British-flagged tanker seized in the Gulf, Foreign Secretary Jeremy Hunt has said.
The Stena Impero's owners say they have been unable to contact their vessel, which had 23 personnel on board and was "heading north towards Iran".
It was surrounded by four vessels and a helicopter, Mr Hunt said.
A second British-owned Liberian-flagged tanker was boarded by armed guards but is now free to continue its journey.
The Mesdar's Glasgow-based Norbulk Shipping UK said communication had been re-established with the vessel after it was boarded by armed guards at around 5.50pm (UK time) on Friday and its crew was "safe and well".
There were 25 personnel on board the Mesdar when it was surrounded by 10 speedboats, Mr Hunt said.
The Mesdar is Liberian-flagged but British owned.
The government's emergency committee, Cobra, is meeting in Whitehall later for the second time on Friday to discuss the incident.
Mr Hunt said the seizures were "unacceptable" and the emergency meeting would review what the UK could do.
"It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region," he added.
He said the tankers' crews were made up of a range of nationalities but no British citizens were understood to be on board either vessel.
"Our ambassador in Tehran is in contact with the Iranian Ministry of Foreign Affairs to resolve the situation and we are working closely with international partners," he said.
These latest developments come amid heightened tensions between the UK, the United States, and Iran.
The Tasnim news agency quoted the Ports and Maritime Organisation of Iran as saying: "We received some reports on the British oil tanker, Stena Impero, causing problems.
"We asked the military forces to guide this tanker towards Bandar Abbas port to have the required investigations carried out."
The Iranian Revolutionary Guard-affiliated news agency said the tanker was seized for breaking three regulations: shutting down its GPS; going through the exit of the Strait of Hormuz rather than the entrance; and ignoring warnings.
Relations between Iran and the West have been increasingly strained since British naval forces seized an Iranian tanker in Gibraltar on 4 July on suspicion of smuggling oil to Syria in breach of European Union sanctions.
On 9 July, the UK raised the threat to British shipping in Iranian waters in the Gulf to "critical" - the highest level.
A day later, Iranian boats attempted to impede a British oil tanker in the region, before being warned off by a Royal Navy ship, according to the Ministry of Defence.
Iran denied any attempted seizure.
Labour deputy leader Tom Watson tweeted that the latest reports were of "real concern", adding that "any move to seize a British tanker would be a significant and harmful escalation of a situation where de-escalation is needed".
Sir Richard Dalton, former UK ambassador to Iran, told the BBC that Iran was "trying to put a scare into the owners and operators of tankers" in the region.
Meanwhile, the US claims to have destroyed an Iranian drone in the Gulf, after Iran shot down an American military drone in the area in June.
US President Donald Trump said he would talk to the UK following the claims about the first tanker.
A White House National Security Council spokesman said it was the second time in just over a week the UK had been "the target of escalatory violence" by Iran.
He added: "The US will continue to work with our allies and partners to defend our security and interests against Iran's malign behaviour."
Iran has been blamed by the US for attacks on tankers which have happened in the world's key shipping area since May. Tehran denies all the accusations.
Tensions between the US and Iran have risen sharply since the US tightened sanctions it reimposed on Iran after unilaterally withdrawing from a landmark 2015 nuclear deal.
- BBC / Reuters