British foreign minister Jeremy Hunt has said he is worried that Iran had taken a "dangerous path", after it seized a British-flagged tanker.
"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," Mr 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 Grace1 issue but WILL ensure the safety of our shipping."
The British navy seized Iran's Grace 1 tanker in Gibraltar on 4 July on suspicion of smuggling oil to Syria in breach of European Union sanctions.
Mr Hunt has also said he is "extremely concerned" by the "unacceptable" seizure of oil tankers by Iranian authorities in the Persian Gulf.
Mr Hunt, who attended a meeting of the British government's emergency committee, Cobra, last night, warned there would be "serious consequences" if the situation was not resolved quickly.
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.
The Foreign Office in London has summoned Iran's charge d'affaires over the incident.
Fars reported that the Stena Impero was in an accident with an Iranian fishing boat and ignored its distress call.
All 23 crew on the tanker were now at Bandar Abbas port and would remain on the vessel until the end of an investigation, Fars quoted the official as saying.
"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 told Fars.
"The tanker is now at Iran's Bandar Abbas port and all its 23 crew members will remain on the ship until the probe is over."
The crew was made up of 18 Indian nationals and five others from Russia, the Philippines, Lithuania and elsewhere, he added.
A second oil tanker, the Liberian-flagged Mesdar, which is managed by Norbulk Shipping UK, veered off course towards the Iranian coast after it was boarded by armed guards at around 5.30pm yesterday, according to the Glasgow-based firm.
The operator said communication had since been re-established with the ship and the crew were unharmed.
In a statement, it said: "Communication has been re-established with the vessel and the 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.
Mr Hunt said: "I'm extremely concerned by the seizure of two vessels by Iranian authorities in the Strait of Hormuz.
"Their crews comprise a range of nationalities, but we understand there are no British citizens on board either ship.
"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."
Mr Hunt later told Sky News: "We will respond in a way that is considered but robust and we are absolutely clear that if this situation is not resolved quickly there will be serious consequences."
Asked if he could rule out military intervention, Mr Hunt said: "We're not looking at military options, we're looking at a diplomatic way to resolve the situation, but we are very clear that it must be resolved.
"Freedom of navigation in the Gulf is absolutely essential. If that freedom of navigation is restricted, Iran is the biggest loser and so it is in their interest to resolve this situation as quickly as possible and we will do everything we can to do that."
He said the Stena Impero was surrounded by four vessels and a helicopter, and is heading into Iranian waters.
The second ship, the Mesdar, was surrounded by ten speedboats, Mr Hunt told Sky, though he said before the meeting it was "not clear yet" whether it had changed course.
He said he had spoken to the US secretary of state, Mike Pompeo, about the situation and had tried to speak to Iranian foreign minister, Mohammad Javad Zarif, but he was on a plane.
President Donald Trump said the US 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 yesterday.
The company said the tanker was in international waters at the time but 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, a Royal Navy warship 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 4 July 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.