A British security guard was among two crew members killed in what a UK source described as a suspected drone attack on an Israeli-linked oil tanker off Oman.
There has not been any official confirmation about what happened to the Mercer Street vessel, which is managed by a British company, and no one has claimed responsibility.
The UK source said that the crew had reported being targeted by "some sort of drone" on Thursday in the Arabian Sea before communications with the ship were lost.
If a drone attack is confirmed it would raise speculation about a possible link to a government or some kind of proxy group.
Iran in the past has repeatedly been accused of targeting tankers in the Gulf.
The country has been locked in a tit-for-tat contest with arch-rival Israel, with both sides accusing the other of targeting ships and launching cyber attacks.
Israel has accused Iran of being behind the attack, and called for a harsh response to the matter.