Suspected attacks on two oil tankers off the coast of Iran on Thursday triggered concerns about global oil prices and the potential for armed conflict between Tehran and its adversaries, including the United States, which has ratcheted up sanctions and threats against the Islamic republic.

Crew members of both supertankers in the Gulf of Oman have been evacuated to safety. But their ships were reportedly stricken and ablaze on the sea, according to photographs and imagery posted by Iranian media.

Both ships had just transited out from the Strait of Hormuz, the narrow waterway abutting Iran and connecting the Gulf to international seaways. The alleged attacks came in quick succession, both ships within 50 miles of each other and within 20 miles of the Iranian coast.

The incident sent shock waves across the world. “I strongly condemn any attack against civilian vessels,” Antonio Guterres, UN secretary general, told the Security Council. “Facts must be established and responsibilities clarified. If there is something the world cannot afford it is a major confrontation in the Gulf region.”

Global oil prices were up 4.5 per cent over worries about the security of tankers exiting the Gulf. The UK Maritime Trade Operations, which is run by the British navy, said it was aware of the incident and investigating, warning ships in the vicinity of the Indian Ocean to exercise “extreme caution”.

The potentially major disruption to Gulf energy traffic came as the Japanese prime minister, Shinzo Abe, made a historic visit to Tehran in an attempt to serve as a mediator between Iran and the Trump administration.

Washington, backed by Saudi Arabia and the United Arab Emirates, has imposed harsh sanctions on Iran over its missile programme and support for armed groups in the Middle East. It has demanded that companies stop buying Iranian energy products or be barred from doing business with the United States.

Tensions high as Trump approves new Iran sanctions: In pictures Show all 17 1 /17 Tensions high as Trump approves new Iran sanctions: In pictures Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, their surface-to-air missile is seen as it shoots down a US surveillance drone EPA Tensions high as Trump approves new Iran sanctions: In pictures This photo shows US RQ-4A Global Hawk unmanned surveillance drone. A drone of this model was shot down by Iran on Thursday 21 June AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures An oil tanker is on fire after it was subject to a suspected attacked at the Gulf of Oman on June 13. The US has blamed Iran for the attack Reuters Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, debris from the downed US drone is seen after it was recovered from Iranian waters AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures US President Trump holds up a signed executive order to increase sanctions on Iran on 24 June AP Tensions high as Trump approves new Iran sanctions: In pictures Iranian President Rouhani stated in a televised address that the White House is "afflicted by mental retardation" following the increase in sanctions on 25 June EPA Tensions high as Trump approves new Iran sanctions: In pictures An Iranian navy boat tackles the fire on the Norwegian owned Front Altair oil tanker after it was hit in a suspected attack AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Iran claims that in this picture released by Iran state TV, debris from the downed US drone is seen after it was recovered from Iranian waters AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures A screenshot from a video released by the US Department of Defense that the US claims to show Iranian removing an unexploded limpet mine form the hull of the Japan-owned ship that was attacked in the Gulf of Oman on June 13 Getty Tensions high as Trump approves new Iran sanctions: In pictures An item reportedly found on the Japan-owned oil tanker that was attacked on June 13 in the Gulf of Oman AFP/Getty Tensions high as Trump approves new Iran sanctions: In pictures A handout photograph from the US Department of Defense shows a composite material that the US claim was left behind on the hull of the Japan-owned oil tanker following the removal of an unexploded limpet mine Getty Tensions high as Trump approves new Iran sanctions: In pictures A view from the cabin of an Iranian navy boat as it tackles the fire on the Norwegian owned Front Altair oil tanker after it was hit in a suspected attack EPA Tensions high as Trump approves new Iran sanctions: In pictures Release by US government A picture released by U.S. Central Command shows damage to the hull of the oil tanker Kokuka Courageous. The picture suggests that the ship is 'likely' to have been hit by a mine as the US Secretary of State Mike Pompeo moves to blame Iran for the suspected attack Reuters Tensions high as Trump approves new Iran sanctions: In pictures Smoke billows from an oil tanker after it was subject to a suspected attacked at the Gulf of Oman on June 13 Reuters Tensions high as Trump approves new Iran sanctions: In pictures Release by US government A picture released by U.S. Central Command shows damage to the hull of the oil tanker Kokuka Courageous. The pictures suggests that the ship is 'likely' to have been hit by a mine as the US Secretary of State Mike Pompeo moves to blame Iran for the suspected attack EPA

Iran’s economy has withered under the sanctions, and the country’s officials have vowed to respond by punishing Washington and its allies.

Japan’s ministry of trade has said one of the ships was bound for the east-Asian country and the incidents are still being examined by maritime and security officials. Mr Abe, the first Japanese premier to visit Iran in more than 40 years, met supreme leader Ali Khamenei on Thursday.

Both ships were close to a major naval base called Jask, and suspicion will fall on Iran over the damage. The attacks could hurt Donald Trump politically by causing a spike in fuel prices just before summer holidays in the US. But Iran’s foreign minister, Javad Zarif, quickly denied his nation’s culpability: “Reported attacks on Japan-related tankers occurred while PM Shinzo Abe was meeting with Ayatollah Khamenei for extensive and friendly talks,” he wrote on Twitter. “Suspicious doesn’t begin to describe what likely transpired this morning.”

There was no claim of responsibility for any attack.

Iranian president, Hassan Rouhani, met Shinzo Abe in Tehran (AFP/Getty) (AFP/Getty Images)

The Kokuka Courageous​, sailing from Saudi Arabia, and the Front Altair, which had loaded fuel in the United Arab Emirates, were heading from the Gulf to ports in Asia. Both sustained damage shortly after transiting from the Strait of Hormuz on Thursday morning, according to news reports and company officials cited by international media.

Both ships sent distress signals that were received by naval forces, including the Bahrain-based US navy’s Fifth Fleet, which responded by dispatching USS Bainbridge to the scene.

Iranian media outlets, believed to be the first or among the first to report any incident, cited sources as saying the damage was caused by explosions in quick succession. The owners of the Kokuka Courageous said it was carrying a cargo of ethanol and that the ship’s hull had been breached “as a result of the suspected attack”.

Confusion and conflicting reports circulated about the ships. Iran’s state broadcaster reported that the Norwegian owned Front Altair, loaded with 75,000 tons of a flammable oil, had sunk before its operator, Frontline, told The Independent that the vessel remained “safely afloat”.

Both US and Iranian navies reportedly responded to the incident by dispatching rescuers. Iran’s official news agency said that Iranian search-and-rescue vessels saved 44 crew members from the ships and brought them to a nearby port.

The tankers Front Altair and Kokuka Courageous’s movements are shown in this still image taken from an animation (MarineTraffic)

Norway and the UAE claimed that an unspecified “state actor” was probably behind attacks on four vessels last month off the coast of Fujairah. US officials have accused Iran of being behind the attacks, but have provided no proof.

Saudi Arabia also accused Iran of being behind a missile attack on Wednesday on a civilian airport in its southern city of Abha that was claimed by Tehran’s Yemeni allies, the Houthi militia, which is in a conflict against Riyadh and its allies.

Aaron Stein, Middle East director at the Foreign Policy Research Institute, told The Independent that Iran was probably the “primary suspect” for Thursday’s attacks.

“It looks like a tit-for-tat in response to the Americans sanctions approach, and it’s calibrated in such a way as to make life uncomfortable – without forcing a response.

“They’ve made the very rational and calculated assumption that the US would not escalate to the point where it would threaten the regime.”

But Iran too can count on allies. Russian deputy foreign minister, Sergei Ryabkov, warned not to pin blame on Iran for the Gulf of Oman incident without any evidence.

“We have been witnessing of late an intensifying campaign of political, psychological and military pressure on Iran,” he was quoted as saying by the Interfax news agency. “We would not like the events that have just happened, which are tragic and which have also shaken the global oil market, to be used speculatively to further escalate the situation in an anti-Iranian way.”

Tokyo, once a significant customer of Iranian energy, enjoys warm relations with Tehran while Mr Abe and Mr Trump have built up a rapport. But early indications suggested Mr Abe was unable to convince Iranian authorities to accede to US demands to reopen negotiations over Iran’s nuclear programme and a range of other issues.