The United Kingdom Maritime Trade Operations (UKMTO) said on Thursday it received a report of an incident 38 nautical miles (70 km) northeast of the United Arab Emirates port of Fujairah.
It said the vessel was boarded by unauthorized personnel while at anchor and is now heading toward Iranian territorial waters.
The incident comes as India said on Thursday that an Indian-flagged ship had been attacked off the coast of Oman on Wednesday, and all crew on board were safe.
"The attack... is unacceptable and we deplore the fact that commercial shipping and civilian mariners continue to be targeted," India's foreign ministry said in a statement.
The statement did not say who had carried out the attack.
Chinese supertanker breaks through Hormuz blockade
A Chinese supertanker sailed through the Hormuz on Wednesday after being stranded in the Gulf for over two months, LSEG and Kpler ship-tracking data showed.
The Very Large Crude Carrier Yuan Hua Hu is now anchored off the Gulf of Oman, near where the US Navy has set up a blockade on Iranian ships, LSEG data showed.
The crossing came before US President Donald Trump and Chinese President Xi Jinping met in Beijing on Thursday, and after Iranian Foreign Minister Abbas Araghchi visited the city last week.
The incident marks the third known passage by a Chinese oil tanker through the Strait of Hormuz since the US-Israel war with Iran began in late February, based on ship-tracking data.