Two oil tankers in the Gulf of Oman, near the strategic Strait of Hormuz, have reportedly been attacked, the US navy said, with one adrift and on fire amid heightened tensions between Washington and Tehran.
Benchmark Brent crude oil spiked 4% in trading following the reported attack to more than $62 a barrel, according to early market figures.
The latest incident comes after the US alleged that Iran used mines to attack four oil tankers off the nearby Emirati port of Fujairah last month.
Iran has denied being involved, but it comes as Iranian-backed rebels in Yemen have also launched missile and drone attacks on Saudi Arabia.
Cmdr Joshua Frey, a 5th Fleet spokesman, said the US navy was assisting the two vessels.
He did not say how the ships were attacked or who was suspected of being behind the assault.
Dryad Global, a maritime intelligence firm, preliminarily identified one of the vessels involved as the MT Front Altair, a Marshall Islands-flagged crude oil tanker.
The vessel was "on fire and adrift", Dryad added.
Iranian state television's website, citing the pro-Iran Lebanese satellite news channel Al-Mayadeen, said two oil tankers had been targeted in the Gulf of Oman.
It offered no evidence to support the claim.
The United Kingdom Maritime Trade Operations, a maritime safety group run by the British Navy, first put out the alert early on Thursday, giving co-ordinates for the incident some 45 kilometres (25 miles) off the Iranian coastline.
The timing of Thursday's reported attack 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, striking the arrivals hall before dawn and wounding 26 people.
Mr Abe met 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.
- Press Association