The US Navy's 5th Fleet says it is assisting two oil tankers targeted in a "reported attack on tankers in the Gulf of Oman".
The Bahrain-based force did not elaborate on who attacked what it described as two oil tankers. It also did not blame anyone for the attack.
The 5th Fleet said it had sent naval forces to the area to assist the two vessels. One has been identified as the MT Front Altair, a Marshall Islands-flagged crude oil tanker.
Japan's Trade Ministry says the oil tankers reportedly attacked near the Strait of Hormuz carried "Japan-related" cargo.
Today's comment 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.
No one has claimed responsibility or explained how the tankers were attacked. However, the US previously blamed Iran for an attack last month on four oil tankers close to the nearby Emirati port of Fujairah.
Meanwhile, Iranian state television reported that 44 sailors from the two tankers have been transferred to an Iranian port in the southern province of Hormozgan.
A firm that operates a crude oil vessel — one of two reportedly targeted in the Gulf of Oman — says an explosion caused a fire onboard.
Another shipping firm identified the second vessel hit and said 21 sailors were evacuated, with one slightly injured in the incident.
International Tanker Management, which operates the MT Front Altair, told The Associated Press the incident is still being investigated and that it was unclear what caused the explosion, which occurred around 8am local time on Thursday.
The MT Front Altair had been loaded at a port in the Gulf with a petroleum product known as naptha, and was on its way to the Far East. Its crew of 23 is safe after being evacuated by the nearby Hyundai Dubai vessel.
Meanwhile, BSM Ship Management says its crew of 21 aboard the Kokuka Courageous carrying methanol has also been rescued in the Gulf of Oman by a nearby vessel after what it described as an "incident on board which resulted in damage to the ship's hull starboard side."
One crew man was slightly injured in the incident and is receiving first aid on board the Coastal Ace.
Today's maritime alert comes after what the US has described as Iranian attacks on four oil tankers nearby, off the coast of the United Arab Emirates.
Iran has denied being involved.