DUBAI, United Arab Emirates (AP) — A ship in the Gulf of Oman has been boarded by “unauthorized” people early Thursday morning, an advisory group run by the British military warned.

Details over the incident remained unclear. The British military’s United Kingdom Maritime Trade Operations, which provides warnings to sailors in the Middle East, said the incident began early Wednesday morning in waters between Oman and Iran in an area transited by ships coming in and out of the Strait of Hormuz, the narrow mouth of the Persian Gulf through which a fifth of all oil traded passes.

The U.K. military-run group described receiving a report from the ship’s security manager of hearing “unknown voices over the phone” alongside with the ship’s captain. It said that further efforts to contact the ship had failed.

The U.S. Navy’s 5th Fleet, which patrols the Mideast, did not immediately respond to a request for comment over the incident. Iran and Oman did not immediately acknowledge the boarding.

Since the collapse of Iran’s nuclear deal, waters around the strait have seen a series of ship seizures by Iran, as well as assaults targeting shipping that the Navy has blamed on Tehran. Iran and the Navy also have had a series of tense encounters in the waterway, though recent attention has been focused on the Iranian-backed Houthi rebels of Yemen attacking ships in the Red Sea amid Israel’s war on Hamas in the Gaza Strip.