Projectiles struck a ship in the narrow Strait of Hormuz, the British military said on Monday.
It marked the second attack in hours as tensions remain high between Iran and the United States.
The British military’s United Kingdom Maritime Trade Operations (UKMTO) centre issued the alert, saying a tanker had been targeted off Fujairah, United Arab Emirates, in the strait around 11.40pm on Sunday.
It described the tanker had been struck by “unknown projectiles”.
“All crew reported safe,” the centre said. “No environmental impact reported.”
Earlier on Sunday, a cargo ship near the strait said it was attacked by multiple small craft, according to the UKMTO.
No injuries were reported.
They were the first reported attacks in the area since April 22. Tehran has effectively closed the strait by attacking and threatening ships and the threat level in the area remains critical.
The first ship was an unidentified cargo ship travelling north near Sirik, Iran, east of the strait, the British monitor said.
Iranian officials have asserted that they control the strait and that ships not affiliated with the United States or Israel can pass if they pay a toll, challenging the freedom of navigation guaranteed by international law.
Iran denied an attack, the semi-official Iranian outlets Fars and Tabnak reported, and said a passing ship had been stopped for a documents check as part of monitoring.
Iranian patrol boats, some powered only by twin outboard motors, are small, nimble and hard to detect. US President Donald Trump last month ordered the US military to “shoot and kill” small Iranian boats that deploy mines in the strait.