A vessel has been struck by an "unknown projectile" while transiting the Strait of Hormuz, the United Kingdom Maritime Trade Operations (UKMTO) said early on Sunday.
The status of the crew, the extent of any damage and the environmental impact were not immediately known, the agency said.
Unlike in most previous incidents, UKMTO did not give the exact location of the attack, leaving it unclear whether the vessel was hit off the coast of Oman or closer to Iran.
It also remained unclear who was responsible.
Iran continues to largely disrupt shipping through the strait with threats and attacks on vessels.
The Strait of Hormuz is a vital route for exports of oil, liquefied natural gas and fertilizer from Gulf states. The disruption has had consequences for the global economy.
The waterway has become a central flashpoint in the war between the United States and Iran. Before the conflict, the strait was open to shipping.