US Secretary of State Mike Pompeo said that the United States government blames Iran for attacks on two oil tankers near the Persian Gulf, casting it as the latest in a series of "provocative actions" that have sharply raised tensions in the region.
A US assessment of Iran's responsibility, which forced the evacuation of the crews in international waters, was based in part on intelligence as well as the expertise needed to carry out the operation, Pompeo told reporters in Washington.
It was also based on a recent series of incidents in the region that the US also blames on Iran, including a similar attack on tankers in the area in May and the bombing of an oil pipeline in Saudi Arabia by Iranian-backed fighters, he said.
"Taken as a whole these unprovoked attacks present a clear threat to international peace and security, a blatant assault on the freedom of navigation and an unacceptable campaign of escalating tension by Iran," Pompeo said.
The US planned to raise the attacks at the UN Security Council later on Thursday (US time). Pompeo also said the US would defend its forces and interests in the Middle East but gave no details on any immediate plans.
Pompeo did not take questions after giving the brief statement.Tensions between Iran and the United States have been growing since President Donald Trump last year withdrew from an international agreement aimed at restricting Iran's nuclear programme and re-instated economic sanctions that have had a devastating effect on the Iranian economy.
In May, the US rushed an aircraft carrier strike group and other military assets to the Persian Gulf region in response to what it said were threats from Iran.
Pompeo on Thursday said Iran had attempted the covert deployment of small boats capable of launching missiles, in an apparent description of the threat that prompted the deployment.
Crews on the tankers were evacuated, a month after a similar incident in which four tankers in the region were struck amid heightened tensions between Washington and Tehran.
One of the ships, the Norwegian-owned Front Altair, was "suspected of being hit by a torpedo", according to Taiwan's state-owned petrol company. The ship was on fire, its owners said.
The second tanker, Kokuka Courageous, said its vessel was on fire following a "suspected attack" in the Gulf of Oman while on passage from Saudi Arabia to Singapore, according to Bernhard Schulte Ship management.
The company said the ship was safely afloat.
The tankers were struck in the same area where the US accused Iran of using naval mines to sabotage four other oil ships in an attack last month.
Russia was quick to urge caution, saying no one should rush to conclusions about Thursday's incident or use it to put pressure on Tehran, which has denied the US accusations.
UN Secretary-General Antonio Guterres said the world cannot afford a major confrontation in the Persian Gulf region.
The United Nations chief told the UN Security Council he is deeply concerned at the "security incident" in the strategic Strait of Hormuz.
"I strongly condemn any attack against civilian vessels," he said. "Facts must be established, and responsibilities clarified."
Japan's Trade Ministry said the two vessels had "Japan-related cargo".
Hiroshige Seko said on Thursday that all crew members were safely rescued.
The attacks came as Japanese Prime Minister Shinzo Abe was wrapping up a two-day trip to Iran with a mission to ease tensions between Tehran and Washington.
Iran's Foreign Minister tweeted in English that "suspicious doesn't begin to describe what likely transpired this morning".
No one has claimed responsibility or explained how the tankers were attacked.
The US Navy said it was assisting the tankers in the Gulf of Oman.
A spokesman for the US Central Command says 21 crew members rescued from the oil tanker Kokuka Courageous are now on board the Navy's USS Bainbridge following an explosion.
Lt. Col. Earl Brown says the US Navy ship was in international waters in the Gulf of Oman near the Courageous when it received a distress call at about 6am local time.
Brown says the Bainbridge provided "immediate assistance" to the Courageous and its crew members after they abandoned ship.
Naval Forces Central Command also received a distress call from the MV Front Altair.
The shipping company that operates the Front Altair said earlier that the crew of that ship was safely evacuated.
Iran's navy rescued 44 crew members from the two oil tankers which caught fire after what was reported as an "accident", official news agency IRNA reported.
"Forty-four sailors from the two foreign oil tankers which had an accident this morning in the Sea of Oman were saved from the water by the (navy) rescue unit of Hormozgan province and transferred to the port of Bandar-e-Jask," IRNA quoted an "informed source" as saying.
Benchmark Brent crude spiked at one point by as much 4 per cent in trading following the reported attack, to over $US62 a barrel, highlighting how crucial the area remains to global energy supplies.
The attacks were near the entrance to the Strait of Hormuz, a crucial shipping artery for Saudi Arabia, the world's biggest oil exporter, and other Gulf energy producers.
"We need to remember that some 30 per cent of the world's (seaborne) crude oil passes through the straits. If the waters are becoming unsafe, the supply to the entire Western world could be at risk," said Paolo d'Amico, chairman of INTERTANKO tanker association.
IRAN: ATTACK ON TANKERS 'SUSPICIOUS'
The latest incident comes after the US alleged that Iran used mines to attack four oil tankers off the nearby Emirati port of Fujairah last month.
Iran has denied being involved, but it comes as Iranian-backed rebels in Yemen also have launched missile and drone attacks on Saudi Arabia.
The Iranian foreign minister described the reported attack as suspicious, since it occurred during a meeting between Japan's prime minister and Iran's supreme leader.
Mohammad Javad Zarif made the comment in a tweet today: "Suspicious doesn't begin to describe what likely transpired this morning." He didn't elaborate.
Zarif described the talks between Shinzo Abe and Ayatollah Ali Khamenei as "extensive and friendly."
However, Khamenei said Iran would never negotiate with the US and said that while his country didn't seek nuclear weapons, "America could not do anything" to stop Iran if it did.
Dryad Global, a maritime intelligence firm, preliminarily identified one of the vessels involved as the MT Front Altair, a Marshall Islands-flagged crude oil tanker.
The vessel was "on fire and adrift," Dryad added.
It did not offer a cause for the incident or mention the second ship.
The firm that operates the Front Altair told The Associated Press that an explosion was the cause of the fire on-board. International Tanker Management declined to comment further saying they are still investigating what caused the explosion. Its crew of 23 is safe after being evacuated by the nearby Hyundai Dubai vessel, it said.
The second vessel was identified as the Kokuka Courageous. BSM Ship Management said it sustained hull damage and 21 sailors had been evacuated, with one suffering minor injuries.
The Japan Shipowners' Association said one of the two ships attacked is a Panamanian-registered chemical tanker belonging to its Japanese member and was on its way to Singapore and Thailand, not to Japan. It said all 21 Filipino crew members were uninjured, alluding to the Kokuka Courageous.
Japan has set up a task force and informed the shipping industry to use precautions.
A third of all oil traded by sea passes through the strait, which is the narrow mouth of the Persian Gulf.
On Wednesday, after talks with Iranian President Hassan Rouhani, Abe warned that any "accidental conflict" that could be sparked amid the heightened US-Iran tensions must be avoided.
His message came just hours after Yemen's Iranian-backed Houthi rebels attacked a Saudi airport, striking its arrivals hall before dawn and wounding 26 people on Wednesday.
Tensions have escalated in the Middle East as Iran appears poised to break the 2015 nuclear deal with world powers, an accord that the Trump administration pulled out of last year.
Iran's nuclear deal, reached in 2015 by China, Russia, France, Germany, the United Kingdom and the US, saw Tehran agree to limit its enrichment of uranium in exchange for the lifting of crippling sanctions.
Western powers feared Iran's atomic program could allow it to build nuclear weapons, although Iran long has insisted its program was for peaceful purposes.