The French foreign ministry has called on Iran to quickly release a British-flagged oil tanker it detained on Friday afternoon.
The Stena Impero and its 23 crew members were approached by unidentified small crafts and a helicopter as it passed through the Strait of Hormuz on Friday at around 4pm UK time, according to the ship's Swedish owners.
France expressed its "full solidarity" with Britain, adding that its was "very concerned" by the seizure of the tanker.
Germany also voiced concern, saying the seizure of the tanker was an "unjustifiable intrusion" on shipping through a key Persian Gulf route that is increasing tensions in the region.
UK foreign minister Jeremy Hunt has warned Iran will face "serious consequences" if it does not release the tanker.
The foreign secretary attended a meeting of the government's emergency committee Cobra on Friday night amid escalating tensions in the Gulf, where the UK frigate, HMS Montrose, is on patrol to boost security.
Mr Hunt said Iran's actions were "completely unacceptable", but stressed he wanted a diplomatic solution to the incident.
Following the talks, a government spokesperson said Iran's actions were "unacceptable" and added that UK ships had been warned to avoid the area.
"We remain in close contact with our international partners and there will be further meetings over the weekend," they said.
The Iranian Revolutionary Guard told state TV last night they had seized the ship for "violating international maritime rules".
A second oil tanker, the British-operated, Liberia-flagged Mesdar, was also boarded by armed Iranian guards before being allowed to continue its voyage, its Glasgow-based owner Norbulk said.
This morning, an Iranian official told Fars News the Stena Impero had been involved in an accident with an Iranian fishing boat and had "ignored" the boat's distress call.
The tanker is reported to be at Iran's Bandar Abbas port, with all 23 crew members still on board. Fars said they would remain there until the end of the investigation. It is understood none are British.
The head of Ports and Maritime Organisation in southern Hormozgan province, Allahmorad Afifipour, told Fars there were 18 Indian crew members and five others from Russia, the Philippines, Lithuania and elsewhere.
MP Bob Seely from the foreign affairs committee told Sky News that the situation with Iran in the Strait of Hormuz is "a massive crisis and it's going to get bigger".
"The Iranians want to internationalise this crisis as much as possible.
"Their economy is hurting and what they want to do in order to get resolution is to widen this as much as possible. The only long-term solution is a Iranian nuclear deal that gets the US back in it. Until then we'll have a series of crises until a really big one blows up."
A statement from the owner of the Stena Impero said the vessel was "in international waters" when it lost contact.
The company, Stena Bulk, said: "Soon after the vessel was approached by unidentified small naval craft and a helicopter during her transit of the Strait of Hormuz in international waters at about 4pm, the vessel suddenly deviated from her passage to Jubail and headed north towards Iran.
"The vessel was in full compliance with all navigation and international regulations".
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 were "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."
But Mr Trump was criticised by Labour leader Jeremy Corbyn, who said America's decision to pull out of the Iran nuclear deal had fuelled tensions in the Gulf.
"A negotiated reinstatement of the nuclear deal through the UN is essential to wind down tensions and defuse the threat of war in the Gulf," he said.
The US says it has authorised the bolstering of forces in Saudi Arabia in response to heightened tensions with Iran.
The Pentagon said it would be sending forces including fighter aircraft, air defence missiles and likely more than 500 troops to a Saudi airbase.
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 shipping within the region whereby no tanker would go in to what is clearly a dangerous zone without an escort, and I find it bizarre that we seem to have ships doing exactly that."
Iran Tensions - Timeline Of Key Events
12 May: Four tankers from the United Arab Emirates, Saudi Arabia and Norway hit by explosives off the Emirati coast.
13 June: Two oil tankers hit by explosions which the US claim were caused by Iranian limpet mines.
20 June: Iranian military shoots down a US surveillance drone.
21 June: Donald Trump calls off a planned airstrike on Iran in retaliation for shooting down the drone.
4 July: Iranian tanker, Grace 1, seized off the coast of Gibraltar for trying to take oil to Syria in violation of European Union sanctions.
11 July: Three Iranian boats try to stop a British oil tanker in the Strait of Hormuz before they were warned off by HMS Montrose.
18 July: US warship destroys an Iranian drone in the Strait of Hormuz after it ignored warnings to back off.
19 July: Iran seizes two oil tankers in the Strait of Hormuz - one the British-flagged Stena Impero, which remains detained. The other vessel, a Liberian-flagged ship, has been released.