US President Donald Trump rejected Iran's insistence that it would not participate in attacks on two oil tankers in the Gulf of Oman.
Mr. Trump quoted images that, according to Washington, show the Iranian forces in a small boat taking an unexploded mine off the hull of one of the ships.
UN Secretary-General Antonio Guterres said the truth must be "clearly established".
Russia warned against "hasty conclusions".
The explosions took place a month after four tankers were damaged during an attack off the UAE. The United States accused Iran of this attack, but they produced no evidence. Iran has also denied these accusations.
Tensions between the United States and Iran have intensified significantly since the inauguration of US President Donald Trump in 2017. He has given up on a nuclear deal with the Obama administration and strengthened it considerably. the sanctions imposed on Iran.
What did Trump say?
Addressing Fox News, he said Iran had "done it".
"I suppose that one of the mines did not explode and that Iran is written on it." And you saw the boat at night trying to pull out the mine and managed to remove the mine from the boat, and this has been exposed, "he said.
He also added that it was unlikely that Iran would close the Strait of Hormuz, a vital shipping route through which one-third of the world's maritime hydrocarbons pass, but if it did so the strait would not remain closed "long".
Mr. Trump's intervention comes after State Secretary Mike Pompeo said that the weapons used, the level of expertise needed to carry out the attacks and that recent similar Iranian attacks on Ships showed that Iran was behind.
US Secretary of Defense Patrick Shanahan said the United States would share intelligence to try to "build an international consensus on this international problem."
How did Iran respond?
Iranian President Hassan Rouhani on Friday accused the United States of constituting "a serious threat to stability in the Middle East", without referring directly to attacks in the Gulf of Oman.
He reiterated an appeal to the international parties to the 2015 nuclear agreement to honor their commitments, following the unilateral withdrawal of the Trump government.
Minister of Foreign Affairs Javad Zarif on Twitter accused the United States to make an allegation "without any factual or circumstantial evidence" and to attempt to "sabotage diplomacy".
The images released Thursday by the United States are rather more convincing than the circumstantial evidence provided previously.
The small white patrol vessel of the video is typical of the one used by the Iranian Navy IRGC (Guardians of the Revolution) in the Gulf. In recent years, the IRGC Navy has gradually supplanted the conventional Iranian navy all along the Gulf Coast of Iran, from its border with Kuwait to the north, to Pakistan and to the sea of Oman. Its forces have been a formidable flotilla of small attack ships armed with mines, missiles, torpedoes and drones, hard to detect.
IRGC commandos routinely engage in covert operations and simulated attacks. Some of their gear has come close to the US Navy's warships in the Gulf in recent years and the risk of a clash at sea remains.
Iran denied any involvement in Thursday's attacks, claiming that they had been perpetrated by someone who was seeking to derail relations between Iran and the international community. There will probably be doubts in Tehran as to the authenticity of this video.
What we know about explosions
According to the story of the United States, US naval forces in the area received distress calls from the Altair Front, belonging to Norway, at 6:12 am (02:12 GMT) and Kokuka Courageous, at Japanese property, at 07:00 am , after explosions, and moved to the area.
The USS Bainbridge observed Iranian naval vessels operating in the area within hours of the explosion, and then removed the unexploded mine from the Kokuka Courageous side.
The crews of both vessels were evacuated to other nearby vessels. Iran and the United States then released images showing crew members saved aboard their ships.
BSM Ship Management, which runs the Kokuka Courageous, said the ship's crew had abandoned the vessel after watching a fire and an unexploded mine.
However, Yutaka Katada, the chairman of the ship's operator, Kokuka Sangyo, said crew members reported "that the ship was attacked by a flying object".
The Kokuka Courageous was about 30 km from the Iranian coast when he sent his urgent appeal.
The Altair Front was carrying naphtha, a petroleum product, from the United Arab Emirates to Taiwan. Kokuka Courageous was carrying methanol from Saudi Arabia to Singapore.
According to the global satellite monitoring company Iceye, the damage to the Altair Front caused an oil spill in the waters surrounding the ship.
Oil prices jumped 4% after Thursday's incident.
Why are US-Iranian tensions so vivid?
In 2018, the United States withdrew from the historic nuclear deal signed in 2015 that aimed to curb Iran's nuclear activities. This decision has been strongly criticized by a number of countries, including the closest allies of the United States.
- US-Iranian tensions explained
In May, President Trump reinforced US sanctions against Iran, targeting mainly the oil sector. Iran has announced that it will suspend certain commitments under the nuclear deal.
In recent months, the United States has strengthened its forces in the Gulf, saying that there was a danger of Iranian attacks. He sent an aircraft carrier strike group and B-52 bombers to the area.
In response, Iran has accused the United States of aggressive behavior. These tensions were sharply accentuated after the attacks on the May 12 mine in the United Arab Emirates.
The UAE has blamed an anonymous "actor of state". The United States said the actor was Iran, denied by Tehran.
Although the reasons why Iran is conducting an attack on multinational tankers are unclear, observers have speculated that it could have sent a signal to the forces deployed against him to point him out. that he was able to disrupt expeditions without causing war.