A US official says an American military team's initial assessment is that Iranian or Iranian-backed proxies used explosives to blow large holes in four ships anchored off the coast of the United Arab Emirates.
The official claims each ship has a 1.5-to-3m hole in it, near or just below the water line.
The US military team assesses that the holes were caused by explosive charges. The UAE asked the US to help investigate the damage, which Gulf officials have characterised as sabotage.
America's Government has warned ships that "Iran or its proxies" could be targeting maritime traffic in the region, and the US has moved additional ships and aircraft into the region.
Two Saudi oil tankers and a Norwegian-flagged vessel were said to be damaged.
While details of the incident remain unclear, it raised risks for shippers in a region vital to global energy supplies at a time of increasing tensions between the US and Iran over its unravelling nuclear deal with world powers.
America is deploying an aircraft carrier and B-52 bombers to the Gulf to counter alleged, still-unspecified threats from Tehran.
A statement from Saudi Energy Minister Khalid al-Falih said the kingdom's two oil tankers, including one due to later carry crude to the US, sustained "significant damage."
A report from Sky News Arabia, a satellite channel owned by an Abu Dhabi ruling family member, showed the allegedly targeted Saudi tanker Al Marzoqah afloat without any apparent damage.
The MT Andrea Victory, another of the allegedly targeted ships, sustained a hole in its hull just above its waterline from "an unknown object," its owner Thome Ship Management said.
Images of the Andrea Victory, which the company said was "not in any danger of sinking," showed damage similar to what the firm described.
Emirati officials identified the third ship as the Saudi-flagged oil tanker Amjad. Ship-tracking data showed the vessel still anchored off Fujairah, apparently not in immediate distress. The fourth ship was the A. Michel, a bunkering tanker flagged in Sharjah, one of the UAE's seven emirates.