There will be "serious consequences" if Iran does not release a British-flagged tanker seized in the Gulf, British 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 operator Norbulk Shipping UK said communication had been re-established with the vessel after it was boarded by armed guards at around 17:30 BST 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 government's emergency committee, Cobra, met in Whitehall twice on Friday to discuss the incident.
A spokeswoman told the BBC the government is "deeply concerned about Iran's unacceptable actions", saying they "represent a clear challenge to international freedom of navigation"."We have advised UK shipping to stay out of the area for an interim period," the spokeswoman added.
Mr Hunt said the seizures were "completely unacceptable" and "freedom of navigation must be maintained".
"We are absolutely clear that if this situation is not resolved quickly there will be serious consequences," he said.
"We are not looking at military options. We are looking at a diplomatic way to resolve this situation but we are very clear that it must be resolved."
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.
The Stena Impero sails under the UK flag and is registered in London.
"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 US, and Iran.
Iranian media reported Stena Impero had been seized by the Iranian Revolutionary Guard.
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.
Stena Bulk, the vessel owner, and Clydebank-based ship manager Northern Marine Management confirmed the UK-registered Stena Impero was approached at around 16:00 BST on Friday while it was in international waters.
A statement said there were no reported injuries and the safety of the crew was the priority of the tanker's owners and managers.
Tensions between the UK and Iran flared up earlier this month when Royal Marines seized an Iranian tanker which was suspected of breaking EU sanctions.
The UK suspected Grace 1, detained on 4 July near Gibraltar, was carrying oil bound for Syria.
In response to the seizure, Iran threatened to seize a British oil tanker.
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.