Saudi Arabia says it intercepted a remotely-controlled boat packed with explosives that targeted its Yanbu port in the Red Sea on Tuesday, with the blast sending black smoke into the sky off the coast.
Private security firms suggested commercial traffic near the port may have been hit in the assault.
Details remained scarce, but the incident comes after a series of attacks on shipping in the wider Middle East region amid a shadow war between Iran and Israel and against the backdrop of ongoing negotiations between Tehran and world powers over Iran's tattered nuclear deal.
The incident also comes amid the six-year-long Saudi Arabian-led intervention against Yemen's Houthi rebels. The Houthis have in the past used bomb-laden drones and explosive-packed boats in attacks targeting the kingdom. However, the rebels did not immediately claim responsibility.
The state-run Saudi Press Agency quoted Saudi military spokesman Col. Turki al-Maliki as saying the port was targeted by the drone boat.
"The booby-trapped boat was dealt with and destroyed according to the rules of engagement," the report quoted al-Maliki as saying, without providing evidence to support his claim.
The United Kingdom Maritime Trade Operations, run by the British navy, simply said it was "aware of reports of an incident" and that investigations were ongoing. Private maritime security firm Dryad Global said it had reports that a ship had been "attacked," without elaborating.
Maritime security firm Neptune P2P Group reported that black smoke was seen billowing near the south entrance of the Yanbu port.
British maritime security firm Ambrey reported the "incident" off western Saudi Arabia, between the ports of Yanbu and Rabigh.
Earlier Tuesday morning, smoke was seen rising from a vessel off the Saudi oil-shipping port of of Yanbu, the firm said. Multiple tankers remain anchored or drifting in the area.
Yanbu port control broadcast a message by marine VHF radio, warning vessels to increase their level of alertness and monitor for any suspicious activity, Ambrey said.
The US Navy's Middle East-based 5th Fleet declined to immediately comment on the incident.
Yanbu, 870km west of Riyadh, serves as the endpoint of the kingdom's crucial East-West Pipeline. It allows crude oil pumped in its eastern fields to be shipped directly via the Red Sea, avoiding the Persian Gulf's chokepoint at the Strait of Hormuz. Yanbu is also home to an oil refinery that can process 400,000 barrels of crude per day.
In May 2019, then-US National Security Adviser John Bolton claimed that Yanbu had been targeted in an attack never acknowledged by the kingdom.
Meanwhile, American and Iranian warships had a tense encounter in the Persian Gulf earlier this month, the first such incident in about a year, the US Navy said Tuesday.
In 2017, the Navy recorded 14 instances of what it describes as "unsafe and or unprofessional" interactions with Iranian forces. It recorded 35 in 2016, and 23 in 2015.
The incidents at sea almost always involve the Revolutionary Guard, which reports only to Supreme Leader Ayatollah Ali Khamenei.
Typically, they involve Iranian speedboats armed with deck-mounted machine guns and rocket launchers test-firing weapons or shadowing American aircraft carriers passing through the Strait of Hormuz, the narrow mouth of the Persian Gulf through which 20 per cent of all oil passes.
Some analysts believe the incidents are meant in part to squeeze President Hassan Rouhani's administration after the 2015 nuclear deal. They include a 2016 incident in which Iranian forces captured and held overnight 10 US sailors who strayed into the Islamic Republic's territorial waters.
The incident comes as Iran negotiates with world powers in Vienna over Tehran and Washington returning to the 2015 nuclear deal, talks due to resume Tuesday. It also follows a series of incidents across the Middle East attributed to a shadow war between Iran and Israel, which includes attacks on regional shipping and sabotage at Iran's Natanz nuclear facility.