Commander Joshua Frey, a 5th Fleet spokesman, said Thursday that the U.S. Navy was assisting two vessels involved in a "reported attack" near the strategic Strait of Hormuz. Washington accused Tehran of being behind a similar attack on May 12 on four tankers in the same area, a vital shipping route through which much of the world's oil passes.
Tensions between Iran and the United States have been growing since President Donald Trump past year withdrew from an worldwide agreement aimed at restricting Iran's nuclear programme and reinstated economic sanctions that have had a devastating effect on the Iranian economy.
Iran has repeatedly warned it would block the Strait of Hormuz, near where the attacks happened, if it can not sell its oil due to US sanctions.
No one has claimed Thursday's attacks and no one has specifically blamed them on any party.
This coincidence did not escape Iran's foreign minister, Javad Zarif, who said, "Suspicious doesn't begin to describe what likely transpired this morning", and reiterated a call for regional dialogue and cooperation.
"An escalation of the situation is unsafe, these are events that could lead to escalation".
The U.S. Navy rushed to assist the stricken vessels in the Gulf of Oman off the coast of Iran, including one that was set ablaze.
The Security Council discussed the attacks behind closed doors on Thursday at the request of the United States.
U.N. Secretary-General Antonio Guterres warned at the meeting that the world can not afford "a major confrontation in the Gulf region".
Two oil tankers were damaged on Thursday in a suspected attack near the entrance to the Persian Gulf, stoking fears that high-stakes diplomatic efforts won't avert a military confrontation between the USA and Iran.
In London, British Foreign Secretary Jeremy Hunt said the United Kingdom was taking the matter "extremely seriously" and that if Iran was involved, "it is a deeply unwise escalation which poses a real danger to the prospects of peace and stability in the region".
The crews of both ships struck on Thursday were picked up safely.
Washington withdrew from the Iranian worldwide nuclear deal in May 2018 and re-imposed full sanctions on Tehran's oil exports and banking sector.
He delivered a note from Trump to Khamenei, but the Ayatollah rebuffed the overture and said he could not expect honest negotiation from the US administration.
Neoconservative Senator Marco Rubio, who has spent the past five months heavily lobbying for United States military intervention in Venezuela, quickly turned his attention to Iran. It is still deniable and denied.
According to the latest World Bank update issued in April, Iran's economy is expected to remain riddled by slow growth and high unemployment for at least the next 12 months "as oil output continues to decline along with other mounting external economic challenges". "It's a blunt message".
"Iran will never initiate a war but will give a crushing response to any aggression", Iranian President Hassan Rouhani said on Wednesday.
Iran said it would not respond to Trump's overture, the substance of which was not made public.
"There are still concerns over geopolitical risk", said Quincy Krosby, chief market strategist at Prudential Financial in Newark, New Jersey.
In Iran, Supreme Leader Ayatollah Ali Khamenei warned that while Tehran doesn't seek nuclear weapons, "America could not do anything" to stop Iran if it did.
"Some parties in the region are trying to instigate fires in the region and we must be aware of that", he told the 15-member council, without specifically naming anyone. A short time later, the Japanese-owned Kokuka Courageous, loaded with methanol from Saudi Arabia and Qatar, also called for help.
Japan's Kokuka Sangyo, owner of the Kokuka Courageous, said the ship was hit twice over a three-hour period.
A United States military handout video purports to show Iran's Revolutionary Guard (IRGC) removing an unexploded limpet mine from the side of the Kokuka Courageous Tanker.
"We're discussing and will be discussing with our partners and allies suggestions on how we collectively can take steps to ensure, one, that we maintain freedom of navigation and worldwide commerce is not disrupted and, second, that we protect our forces' interests and our commercial assets and those of our partners and allies", the official said.
The ship was carrying 75,000 tonnes of naphtha, a petrochemical feedstock, "when it was suspected of being hit by a torpedo", he said.
The ship's owner, Frontline, said the Marshall Islands-flagged vessel was on fire but denied reports on Iran media it had sunk.
All 21 crew members abandoned ship and one member was slightly injured.
Iranian state television said 44 sailors from the two tankers were transferred to an Iranian port in the southern province of Hormozgan. The numbers in the Iranian media report could not be independently confirmed. There was one woman among them.
Thursday's attacks came a day after Yemen's Iran-aligned Houthis fired a missile on an airport in Saudi Arabia, injuring 26 people.
On May 14, Yemen's pro-Iranian Huthi rebels carry out drone attacks near Riyadh, shutting down a key Saudi oil pipeline.