The British agency United Kingdom Maritime Trade Operations (UKMTO) said a tanker was struck by an “unknown projectile” as it entered the Strait of Hormuz on Monday; two crew members suffered minor injuries and the vessel continued to its next port under its own power, with no environmental impact reported.

UKMTO did not release the vessel’s identity or the origin of the projectile and said authorities are investigating, according to the Associated Press (AP) and the Anadolu Agency. A bulletin from the UKMTO-linked Joint Maritime Information Center (JMIC) lists four security incidents involving merchant vessels in the strait between September 14 and 18, three of them caused by unidentified projectiles.