Jeremy Hunt has said he is "extremely concerned" by the "unacceptable" seizure of oil tankers by Iranian authorities in the Persian Gulf, as it emerged one of the ships had been released.
The Foreign Secretary confirmed that he will attend a meeting of the Government's emergency committee Cobra on Friday night amid the escalation of tensions.
The Stena Impero, which is registered in the UK, was seized by the Iranian Revolutionary Guard in the Strait of Hormuz for "violating international maritime rules", according to the semi-official Fars news agency.
And a second oil tanker, the Liberia-flagged Mesdar, which is owned and operated by Glasgow-based firm Norbulk, appeared to veer off course towards the Iranian coast, according to its path on the Marine Traffic website.
But the Mesdar's operator said communication had since been re-established with the ship after it was boarded by armed guards at about 5.30pm on Friday.
In a statement they said: "Communication has been re-established with the vessel and master confirmed that the armed guards have left and the vessel is free to continue the voyage. All crew are safe and well."
Mr Hunt said he understood that there were no British citizens on board either ship.
He said: "I'm extremely concerned by the seizure of two vessels by Iranian authorities in the Strait of Hormuz.
"I will shortly attend a COBR meeting to review what we know and what we can do to swiftly secure the release of the two vessels – a British-flagged vessel and a Liberian-flagged vessel.
"Their crews comprise a range of nationalities, but we understand there are no British citizens on board either ship.
"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.
"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."
US President Donald Trump said America would be "working with the UK".
He told reporters: "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.
"So we heard about it, we heard it was one, we heard it was two, and we will be working with the UK."
A statement from Stena Bulk, which owns the Stena Impero, said ship manager Northern Marine Management had lost contact with the crew of 23 after "unidentified small crafts and a helicopter" approached the vessel at about 4pm on Friday.
The company said the tanker was in international waters at the time but now appeared to be heading north towards Iran.
Stena Bulk said: "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.
"We are in close contact with UK Government authorities."
The incident follows on from recent heightened tensions in the Gulf involving Iran, the US and UK.
Last week, the Royal Navy warship frigate HMS Montrose drove off three Iranian vessels which tried to stop the commercial ship British Heritage as it sailed through the Strait of Hormuz.
Fears were raised that the Iranian authorities were trying to seize a UK ship in retaliation for the detention of the Grace 1 tanker.
The Iranian ship was detained off the coast of Gibraltar on July 4 after it was suspected of violating EU sanctions by carrying a cargo of crude oil destined for Syria.
The ship's captain, chief officer and two second officers were arrested and bailed and an investigation is ongoing.
Iran's supreme leader Ayatollah Ali Khamenei called the tanker's seizure an act of "piracy" on Tuesday and warned the UK to expect a response.
Mr Hunt offered to help release Grace 1 if Iran guaranteed it would not breach sanctions imposed on Bashar Assad's regime.
European allies to the US have been urged to take a tougher stance on Iran after Mr Trump withdrew from a nuclear deal with Tehran.
On Thursday, Mr Trump said a US warship shot down an Iranian drone in the Strait of Hormuz – something denied by Iranian military officials.
Last month, Mr Trump said he had made a last-minute decision to call off air strikes in retaliation for the shooting down by Iran of an unmanned US drone.