Armed men boarded a ship off the Iranian coast on Tuesday (April 14th) before letting it continue its course, a UK-based maritime organisation said.
UK Maritime Trade Operations (UKTMO) sounded the alarm after the boat was boarded while anchored off Iran's south-east coast in the Strait of Hormuz.
The vessel was later released under the control of the ship's captain and vessel and crew were safe, the UKMTO reported.
"All vessels in the vicinity are to stay vigilant and to report any incidents," it said, adding that "vessels transiting the area are advised to exercise caution".
No one claimed responsibility for the seizure, the Associated Press (AP) reported, but suspicion fell on Iran's Islamic Revolutionary Guard Corps (IRGC).
Iranian officials and state media did not immediately acknowledge the incident and the US Navy's Bahrain-based 5th Fleet did not respond to a request for comment, the AP report said.
The incident comes after a private maritime intelligence firm warned of suspicious incidents in recent days near the strait, the news agency added.
Maritime security analysts Dryad Global reported the same incident, saying there were 22 crew aboard the Hong Kong-flagged ship who were understood to be Chinese nationals.
Tensions remain high in the region after Iran was accused by the US in June last year of attacking oil tankers in the Gulf of Oman, a claim denied by Tehran.
Those claims came during a time of increased diplomatic tensions between the US and Iran.