A tanker was struck by an “unknown” projectile off the coast of Fujairah, United Arab Emirates, early on Tuesday morning, according to U.K. Maritime Trade Operations, a marine tracker.

The ship, which was anchored at the time, was about 23 nautical miles off the coast, UKMTO said in a warning.

UKMTO said in an advisory late on Monday that the threat environment remained critical near the Strait of Hormuz, a vital shipping route that Iran has threatened to attempt to close to most marine traffic.

“Since the start of the hostilities on 28 February, at least twenty maritime incidents involving commercial vessels and offshore infrastructure have been reported in the Arabian Gulf, Strait of Hormuz, and Gulf of Oman,” UKMTO said in the advisory.