Dozens of crew members have been rescued after abandoning two oil tankers hit by explosions in the Gulf of Oman.
Iran said it rescued the 21 crew members on board the Kokuka Courageous and the 23 on the Front Altair, though the US said its Navy had rescued some.
The cause of the blasts in one of the world's busiest oil routes is unclear and both vessels are still afloat.
The incident comes a month after four oil tankers were attacked off the UAE.
Oil prices rose as much as 4.5% from a near five-month low following Thursday's incident, Bloomberg reports.
The cause has not been confirmed.
The Norwegian-owned Front Altair had been "attacked", the Norwegian Maritime Authority said, leading to three explosions on board.
Wu I-fang, a spokesman for Taiwan's state oil refiner CPC Corp, which chartered the Front Altair, said it was carrying 75,000 tonnes of naphtha and was "suspected of being hit by a torpedo", although this has not been confirmed. Other unverified reports suggested a mine attack.
The ship's owner, Frontline, said the Marshall Islands-flagged vessel was on fire but denied reports on Iran media it had sunk.
The operator of the Panama-flagged Kokuka Courageous, BSM Ship Management, said its crew abandoned ship and were rescued by a passing vessel.
The tanker was carrying methanol and was not in danger of sinking, a spokesman said.
It is currently located about 80 miles from Fujairah in the UAE and 16 miles from Iran. The cargo remains intact.
This is the second serious incident in a month involving tanker shipping close to the strategic Strait of Hormuz, through which at least one-fifth of the world's oil passes.
In May, four tankers were hit by explosive devices close to a port in the United Arab Emirates. A report presented jointly by the UAE, Saudi Arabia and Norway last week to the UN Security Council concluded they had been placed there by frogmen, but did not assign blame to any country.
But in that incident the damage was minor, there were no fires and no-one was evacuated. This is far more serious.
Both of the stricken Norwegian and Japanese-owned tankers on Thursday were under way and moving out of the Gulf of Oman, prompting questions as to what could have caused so much damage that the crew needed evacuating. Iran has said it is suspicious this incident took place just as it was hosting Japan's Prime Minister Shinzo Abe.
Iranian state media said Iran had rescued the crew members and they had been taken to the port of Jask.
The 21 crew on the Kokuka Courageous (L) and 23 on the Front Altair have been rescued
The US 5th Fleet, based in Bahrain, said it had sent the USS Bainbridge to assist.
Spokesman Josh Frey said in a statement: "US naval forces in the region received two separate distress calls at 06:12 local time (03:12 GMT) and a second one at 07:00."
CNN quoted a defence official as saying some mariners were transferred from a tug to the Bainbridge and then taken to Oman.
The Gulf of Oman lies at one end of the strategic Strait of Hormuz, and this incident will further increase tension in a vital shipping lane through which hundreds of millions of dollars of oil pass.
The US sent an aircraft carrier strike group and B-52 bombers to the region at the start of May in response to what it said was an unspecified plan by Iran-backed forces to attack US forces in the area.
President Donald Trump has taken a hard line towards Iran, accusing it of being a destabilising force in the Middle East.
Iran rejected the claims and has accused the US of aggressive behaviour.
Those tensions rose markedly after the 12 May limpet mine attacks in the UAE.
The UAE blamed an unnamed "state actor". The US said that actor was Iran, an accusation Tehran has denied.
The EU called for "maximum restraint" following Thursday's incident.
Paolo d'Amico, chairman of the tanker association, Intertanko, said the two vessels had been attacked, and expressed concern about dangers to other crews.
"If the waters are becoming unsafe, the [oil] supply to the entire Western world could be at risk," he said.