Jeremy Hunt has warned Iran will face "serious consequences" if it does not release a British-flagged tanker after two ships were seized in the Gulf.
The foreign secretary attended a meeting of the Government's emergency committee Cobra on Friday night amid the escalation of tensions in the region.
Owners of the Stena Impero confirmed the vessel was approached by unidentified small crafts and a helicopter as it passed through the Strait of Hormuz at around 4pm UK time.
A second oil tanker, the Liberia-flagged Mesdar was also boarded by armed Iranian guards before being allowed to continue its voyage, its Glasgow-based owner Norbulk said.
Mr Hunt said Iran's actions were "completely unacceptable".
"We will respond in a way is considered but robust, and we are absolutely clear that if this situation is not resolved quickly there will be serious consequences," he said.
He added: "We're not looking at military options, we are a looking at diplomatic way to resolve the situation.
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"But we are very clear that it must be resolved."
Mr Hunt said he understood that there were no British citizens on board either ship.
A statement from the owner of the Stella Impero said it was "in international waters" when it lost contact.
The company added: "We are presently unable to contact the vessel which is now heading north towards Iran.
"There are 23 seafarers aboard. There have been no reported injuries and their safety is of primary concern to both owners and managers.
"The priority of both vessel owner Stena Bulk and ship manager Northern Marine Management is the safety and welfare of the crew."
The Iranian Revolutionary Guard told state TV they had seized the Stena Impero for "violating international maritime rules".
Iran's semi-official Tasnim news agency said Iran's Revolutionary Guards had not captured the Mesdar.
"Despite reports, the ship has not been seized…and was allowed to continue its course after being warned about safety
issues by Iranian forces," the report said.
A spokesman for Norbulk Shipping UK confirmed the crew of the Mesdar are "safe and well" and the vessel has been "allowed" to continue its voyage.
It comes after the Iranian tanker, Grace 1, was seized on 4 July off the coast of Gibraltar for trying to take oil to Syria in violation of European Union sanctions.
Iran's supreme leader Ayatollah Ali Khamenei called the tanker's seizure an act of "piracy" and warned the UK to expect a response.
Just days later a Royal Navy warship deterred Iranian patrol boats attempting to impede a British tanker sailing through the Strait of Hormuz, leading the government to deploy a second vessel to the region.
US President Donald Trump has announced he will discuss the situation with the British government.
"We will talk to the UK and we have no written agreement but we have an agreement. They've been a very great ally of ours.
"They will have a new prime minister soon, which is a good thing, and we will be working with the UK – but we have no written agreement, but I think we have an agreement which is longstanding."
Lord West, former head of the Royal Navy, told Sky News the UK should not "pretend we're surprised" by the Stena Impero incident.
"What I find extraordinary is that we knew that the Iranians would try something like this a few days ago.
"I'm absolutely amazed that we haven't implemented some sort of control of red ensign sRead More – Source