A remotely piloted boat packed with explosives has targeted the Saudi port of Yanbu in the Red Sea, the kingdom said, with the blast sending black smoke into the sky off the coast.

Saudi Arabia claimed to have intercepted and destroyed the attack boat, but 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 negotiations between Tehran and world powers over Iran’s tattered nuclear deal.

The incident also came amid the kingdom’s years-long war against Yemen’s Houthi rebels. The Houthis have in the past used bomb-laden drones and explosive-packed boats in attacks targeting the kingdom.

The state-run Saudi Press Agency quoted Saudi military spokesman Colonel Turki al-Maliki saying the port was targeted by a drone boat.

“The booby-trapped boat was dealt with and destroyed according to the rules of engagement,” the report quoted him as saying.

The United Kingdom Maritime Trade Operations, run by the Royal Navy, said it was “aware of reports of an incident” and investigations were continuing.

Private maritime security firm Dryad Global said it had reports that a ship had been attacked, without elaborating.

Maritime security company Neptune P2P Group reported that black smoke was seen billowing near the south entrance of Yanbu port.

British maritime security firm Ambrey reported an incident off western Saudi Arabia, between the ports of Yanbu and Rabigh. Smoke was seen rising from a vessel off the oil shipping port of Yanbu, the firm said. Multiple tankers remained 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 port, 540 miles west of Riyadh, serves as the end point of the kingdom’s crucial East-West Pipeline, allowing crude oil pumped in its eastern fields to be shipped directly via the Red Sea, avoiding the Persian Gulf bottleneck at the Strait of Hormuz.

Yanbu is also home to an oil refinery that can process 400,000 barrels of crude per day.

The incident came as the US navy released footage of a tense encounter between American and Iranian warships in the Persian Gulf earlier this month, the first such incident in about a year.

The footage showed a ship commanded by Iran’s paramilitary Revolutionary Guard cut in front of the USCGC Monomoy on April 2, causing the Coast Guard vessel to come to an abrupt stop with its engine smoking.

The Guard did the same with another Coast Guard vessel, the USCGC Wrangell, said Commander Rebecca Rebarich, a 5th Fleet spokeswoman.

Iran did not immediately acknowledge the incident in the southern reaches of the Persian Gulf, which resulted in no injuries or damage.