Burning in waters between Gulf Arab states and Iran - two oil tankers were hit in suspected attacks.
This was carrying a cargo of petrochemical feedstock, now ablaze.
The hit sending oil prices as much as 4% higher a month after four other tankers were damaged by naval mines in the region.
The Norwegian owner of the burning ship said its crew were safe.
While the other tanker owned by Japan was adrift with no one on board.
Full details about Thursday's (June 13) incident were not immediately clear.
The firm which chartered one of the vessels said it suspected a torpedo had hit the ship, while a source said the other might have been damaged by a magnetic mine.
The Bahrain-based U.S. Navy Fifth Fleet said it was assisting the tankers after receiving distress calls.
While the UK Maritime Trade Operations, part of Britain's Royal Navy, said it was investigating with its partners.
Saudi Arabia and the U.S. blamed Iran for last month's tanker attacks, a charge they deny.
The region was already on edge following attacks in May on Gulf oil assets.
That occurred amid a dispute between Iran and the U.S. over Tehran's nuclear programme.
The attacks sending shockwaves through the industry.
With growing concerns for the safety of ships and their crew sailing through the Strait of Hormuz.
The world's most important oil artery.