Four commercial ships were targets of a "sabotage attack" off the United Arab Emirates (UAE) on Sunday, the country's foreign affairs ministry has said.
The incident reportedly took place near Fujairah port, just outside the Strait of Hormuz, but caused no casualties.
Saudi Arabia confirmed that two Saudi oil tankers sustained "significant damage".
Another vessel was Norwegian-registered, whilst the fourth was reportedly UAE-flagged.
Iran, which borders the strait, has called for a full investigation.
Tensions are high in the region. About a fifth of oil that is consumed globally passes through the area.
Last month, Iran threatened to "close" the Strait of Hormuz if it was prevented from using the waterway.
This followed a US decision to end exemptions from sanctions for major importers of Iranian oil.
The US has deployed warships to the region in recent days to counter what it called "clear indications" of threats from Iran to its forces and maritime traffic. Iran dismissed that allegation as nonsense.
A US military official told the BBC that, at the UAE's request, America had send "a team of experienced investigators" to help determine the cause of the damage.
US Secretary of State Mike Pompeo made an unplanned visit to Brussels on Monday to discuss Iran with European foreign ministers.
What do we know about the incident?
Few details have been released about the incident, which is said to have taken place at about 06:00 (02:00 GMT) on Sunday within the UAE's territorial waters in the Gulf of Oman, east of the emirate of Fujairah.
The UAE foreign ministry said on Sunday that it was investigating.
The ministry also denied as "baseless and unfounded" media reports of fires and explosions at Fujairah port, and insisted operations at the facility were normal.
On Monday morning, the official Saudi Press Agency quoted the country's energy minister, Khalid al-Falih as saying two Saudi oil tankers had been among the ships targeted.
"One of the two vessels was on its way to be loaded with Saudi crude oil from the port of Ras Tanura, to be delivered to Saudi Aramco's customers in the United States," he added.
Industry sources told the BBC that the Saudi tankers affected were the Amjad and Al Marzoqah.
Saudi TV has shown images of the damaged Saudi ships, and a picture released by the UAE shows a Norwegian-flagged vessel, Andrea Victory, with damage to its hull.
Thome Ship Management, a Norwegian firm which manages the ship, said in a statement that it had been "struck by an unknown object on the waterline" while anchored off Fujairah. No crew members were harmed.
News agency Reuters reports that the fourth vessel is the UAE-flagged A. Michel, a fuel bunker barge.
Intertanko, an association of independent tanker owners and operators, said it had seen pictures showing that "at least two ships have holes in their sides due to the impact of a weapon", Reuters news agency reported.
Murky Gulf waters
Whoever was behind this offshore incident, it adds yet one more layer to the increasingly complex strategic picture in the Gulf.
The White House accuses Iran of threatening its interests in the Gulf - without saying how exactly - and has dispatched an aircraft carrier group and amphibious assault ship to the Gulf, hoping to intimidate Tehran. The US has also sent a squadron of B52 bombers to an airbase in Qatar, a country that actually enjoys friendly relations with Iran.
The Iranians, for their part, have hinted that "a third country" could be to blame - historically, that has tended to mean Israel - and have called for a full investigation.
Oil tankers have been attacked in the region before, by al-Qaeda off Yemen in 2002, by Somali maritime pirates in the Gulf of Oman, and more recently by Houthi rebels in the Red Sea.
Until verified facts emerge it will be hard to ascertain who is to blame in this case.