UK Foreign Secretary Jeremy Hunt told Parliament that the UK would seek to put together a European-lead maritime protection mission to ensure safe passage through the Strait of Hormuz in response to Iran's seizure of the British-flagged tanker Stena Impero last week.
A British destroyer will arrive in the Strait of Hormuz to help protect vessels next week, and the UK is raising its security level for British-flagged shipping to advise against all passage through Iranian waters and "for the moment" in the entire Strait of Hormuz, Hunt said.
He specified that the decision was not part of the US "maximum pressure" campaign against Iran.
The Stena Impero had been heading to a port in Saudi Arabia when it was seized by Iran's Revolutionary Guard Corps (IRGC) on Friday.
It was "approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz," the company that owns the tanker, Stena Bulk, said.
A video emerged on Saturday purporting to show troops descending onto the tanker. The footage, released by the IRGC, showed a vessel with Stena Impero written down the side.
An audio clip provided by maritime security firm Dryad Global revealed the British navy's HMS Montrose warning Iran's Revolutionary Guard as it seized the tanker.
Hunt said on Monday that HMS Montrose, a royal navy frigate, had tried to come to the vessel's aid, but was unable to reach the scene in time.
A second Liberian-flagged vessel was also boarded by armed personnel on Friday, the ship's technical manager, the UK company Norbulk Shipping said, but the vessel was later released.
Neither of the two vessels had any British citizens on board, the UK foreign office confirmed.
UK foreign secretary Jeremy Hunt said on Saturday that Iran may be choosing a "dangerous path of illegal and destabilising behaviour".
Iran's envoy to Britain on Sunday urged the UK to contain "domestic political forces" which he said wanted to escalate tension between the two countries amid the capture of a British-flagged tanker in the Gulf by Iran.
When asked whether London was considering putting sanctions on Tehran, junior defence minister Tobias Ellwood said on Sunday that Britain is looking into "a series of options" to respond to Iran.
"Our first and most important responsibility is to make sure that we get a solution to the issue to do with the current ship, make sure other British-flagged ships are safe to operate in these waters and then look at the wider picture," Ellwood told Sky News. "We are going to be looking at a series of options... We will be speaking with our colleagues, our international allies, to see what can actually be done."
Iran's Fars news agency, quoting an official, reported on Saturday the British tanker was in an accident with a fishing boat before it was detained.
"It got involved in an accident with an Iranian fishing boat... When the boat sent a distress call, the British-flagged ship ignored it," the head of Ports and Maritime Organisation in southern Hormozgan province, Allahmorad Afifipourm told Reuters.
"The tanker is now at Iran's Bandar Abbas port and all of its 23 crew members will remain on the ship until the probe is over."
The president and chief executive of Stena Bulk, Erik Hanell said he had issued a formal request to visit the 23 crew members. The company said their "insurers in the region" had been in touch with the Iranian port's head of marine affairs who said the crew were in "good health".
The seizure came just two weeks after the UK seized an Iranian tanker, the Grace 1, in Gibraltar on suspicion of smuggling oil to Syria in breach of European Union sanctions. On Monday, Jeremy Hunt said that there was no comparison between that act and Iran's seizure of the Stena Impero.
UK plans for a larger presence in the region
The UK foreign minister Jeremy Hunt announced new measures to protect freedom of navigation in the Strait of Hormuz.
He announced on Monday that the UK would raise its security level to advise against passage in Iranian waters and "for the moment" in the Strait of Hormuz. Hunt said that a UK destroyer would arrive in the region next week to help protect British-flagged vessels in the region.
He also said the UK would seek to create a European-led maritime protection mission to ensure safe passage in the Strait. He specified that the UK reaction was not part of the US "maximum pressure" campaign against Iran and that the UK remained committed to the Iran nuclear agreement.
Iran's chargé d'affaires was summoned to the Foreign Office on Saturday for a formal protest.
Hunt said it was with a "heavy heart" that he announced the increased presence in the region "because the focus of our diplomacy has been on de-escalating tensions."
"We must also react to the world around us as it is and not how we would wish it to be," Hunt said of the government's decision.
Roughly 1,300 ships appear on the UK's ship register, the foreign secretary said.
"Yesterday's action in Gulf shows worrying signs Iran may be choosing a dangerous path of illegal and destabilising behaviour after Gibraltar's LEGAL detention of oil bound for Syria," Hunt said on Twitter.
"As I said yesterday our reaction will be considered but robust. We have been trying to find a way to resolve Grace 1 issue but WILL ensure the safety of our shipping."
Hunt said on Monday that the government had held several emergency meetings and had issued a formal protest to Iran.
France, Germany, Oman call for de-escalation
European countries also reacted to the escalation in the Strait of Hormuz.
Germany on Saturday condemned the "unjustifiable" seizure by Iran of the two vessels and said the move dangerously exacerbated an already tense situation in the area.
"We urge Iran to release (this)... ship and its crew immediately," a German foreign ministry spokesman said.
A further escalation of regional tensions "would be very dangerous", the spokesman said in a statement, and would "undermine all ongoing efforts to find a way out of the current crisis."
France's foreign ministry said on Saturday it was very concerned by the seizure of the British vessel, saying such an action harmed de-escalation efforts in the region.
"We have learned with great concern of the seizure of a British vessel by Iranian forces," the ministry said in a statement. "We strongly condemn it and express our full solidarity with the United Kingdom."
Omani state media reported on Sunday that the country is "following with concern maritime developments in Strait of Hormuz" and calling for "restraint". It added that Oman is calling for Iran to release the British vessel and for both parties to resolve differences diplomatically.
Rising tensions in the Strait of Hormuz
Tensions between Iran and the United States and its allies have risen since Washington imposed new sanctions against Iran as part of a "maximum pressure" campaign.
Three Iranian vessels tried to block a British-flagged oil tanker in the Strait of Hormuz two weeks ago after the British helped to seize Grace 1 in Gibraltar on July 4. The royal navy escorted the oil vessel through the Strait.
The UK defence secretary Penny Mordaunt said in a statement last week that the government urged "Iranian authorities to de-escalate the situation".
Iran has responded by scaling back its commitments under the 2015 nuclear deal.
The Strait of Hormuz is a key strategic location for the global oil trade.