A UK maritime safety group has warned of an unspecified incident in the Gulf of Oman and urged ‘extreme caution’.
The United Kingdom Maritime Trade Operations, which is run by the Royal Navy, put out the alert but did not elaborate on the incident.
It said it was investigating.
Cmdr Joshua Frey, a spokesman for the US navy's Bahrain-based 5th Fleet, said his command was ‘aware’ of a reported incident in the area and they are ‘working on getting details’.
The co-ordinates offered for the incident by the UK group put it some 45 kilometres (25 miles) off the Iranian coastline.
The timing was especially sensitive as Japanese Prime Minister Shinzo Abe was visiting Iran on a high-stakes diplomacy mission.
On Wednesday, after talks with Iranian President Hassan Rouhani, Mr Abe warned that any ‘accidental conflict’ that could be sparked amid the heightened US-Iran tensions must be avoided.
His message came just hours after Yemen's Iranian-backed Houthi rebels attacked a Saudi airport, wounding 26 people.
Mr Abe was to meet with Iran's Supreme Leader Ayatollah Ali Khamenei on Thursday, the second and final day of his visit.
Meanwhile, in Tokyo, Japan's chief cabinet secretary Yoshihide Suga, a top government spokesman, told reporters that Mr Abe's trip was intended to help de-escalate tensions in the Middle East - but not specifically mediate between Tehran and Washington.
His remarks were apparently meant to downplay and lower expectations amid uncertain prospects for Mr Abe's mission.
Tensions have escalated in the Middle East as Iran appears poised to break the 2015 nuclear deal with world powers, an accord that the Trump administration pulled out of last year.