As many as four ships anchored in the Persian Gulf were damaged in what Gulf officials described Monday as a "sabotage" attack off the eastern coast of the United Arab Emirates.

A U.S. official says an American military team's initial assessment is that Iranian or Iranian-backed proxies used explosives to blow large holes in the ships.

The official, who was not authorized to discuss the investigation publicly and spoke on condition of anonymity, said each ship has a 1.5- to three-metre hole in it, near or just below the water line.

The official agreed to reveal the findings only if not quoted by name. No specific evidence was offered.

The U.A.E. asked the U.S. to help investigate the damage to the vessels, which included two Saudi oil tankers.

A damaged Andrea Victory ship is seen off the Port of Fujairah on the eastern seaboard of U.A.E. (Satish Kumar/Reuters)

The U.A.E. identified the Saudi tankers as the Amjad, a very large crude carrier (VLCC), and a crude tanker called Al Marzoqah, both owned by Saudi shipping firm Bahri. The other two vessels were U.A.E.-flagged fuel bunker barge A. Michel and Norwegian-registered oil products tanker MT Andrea Victory.

Thome Ship Management said its MT Andrea Victory was "struck by an unknown object." Footage seen by Reuters showed a hole in the hull at the water line with the metal torn open inward.

Citing heightened tensions in the region, the United Nations called on "all concerned parties to exercise restraint for the sake of regional peace, including by ensuring maritime security" and freedom of navigation, UN deputy spokesperson Farhan Haq said.

Oil markets react

The U.S. had already warned ships last week that "Iran or its proxies" could be targeting maritime traffic in the region, and the U.S. has moved additional ships and aircraft into the region.

The incident raised questions about maritime security in the U.A.E., home to Dubai's Jebel Ali port, the largest man-made deep-water harbour in the world that is also the U.S. navy's busiest port of call outside of America. From the coast, AP journalists saw an Emirati coast guard vessel patrolling near the area of one of the Saudi ships in Fujairah, some 210 kilometres northeast of Dubai on the Gulf of Oman.

Fujairah also is about 140 kilometres south of the Strait of Hormuz, the narrow mouth of the Persian Gulf through which a third of all oil at sea is traded. The alleged sabotage caused jitters in global oil markets, as benchmark Brent crude rose in trading to more than $71.50 US a barrel Monday, a change of 1.3 per cent.

The announcement of the Sunday attack off the coast of the port city of Fujairah came Monday from the kingdom's energy minister, Khalid al-Falih, as the U.S. issued a new warning to sailors.

The U.A.E.'s state news agency said Fujairah port was operating normally. A witness who spoke with Reuters said divers were inspecting damaged ships.

On Twitter, the Saudi Foreign Ministry said: "We condemn the acts of sabotage, which targeted commercial and civilian vessels near the territorial waters of the United Arab Emirates."

The ministry later put out another tweet, saying it stands in solidarity with the U.A.E.

<a href="https://twitter.com/hashtag/STATEMENT?src=hash&ref_src=twsrc%5Etfw">#STATEMENT</a> | WE STAND IN SOLIDARITY WITH THE UNITED ARAB EMIRATES IN ALL MEASURES TAKEN TO SAFEGUARD ITS SECURITY AND INTERESTS 🇸🇦🇦🇪 <a href="https://t.co/s2ityYIsTC">pic.twitter.com/s2ityYIsTC</a> —@KSAmofaEN

The U.S. is deploying an aircraft carrier and B-52 bombers to the Persian Gulf to counter alleged threats from Tehran.

Shortly after the Saudi announcement, Iran's Foreign Ministry called for further clarification about what exactly happened with the Saudi tankers. The ministry's spokesperson, Abbas Mousavi, was quoted by the official IRNA news agency as saying there should be more information about the incident.

Al Marzoqah tanker is seen near the port. (Satish Kumar/Reuters)

Mousavi also warned against any "conspiracy orchestrated by ill-wishers" and "adventurism by foreigners" to undermine the maritime region's stability and security.

Rising tensions in region

Tensions have risen in the year since U.S. President Donald Trump withdrew the United States from the 2015 nuclear deal between Iran and world powers, restoring American sanctions that have pushed Iran's economy into crisis. Last week, Iran warned it would begin enriching uranium at higher levels in 60 days if world powers failed to negotiate new terms for the deal.

European foreign ministers meeting in Brussels urged the United States and Iran to show restraint Monday amid fears of tensions tipping them easily into armed conflict, while Secretary of State Mike Pompeo briefed his counterparts on the threats Washington sees emanating from the Islamic republic.

In his statement, al-Falih said the attacks on the two tankers happened at 6 a.m. local time Sunday.

"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. Fortunately, the attack didn't lead to any casualties or oil spill; however, it caused significant damage to the structures of the two vessels."

Khalid al-Falih, Saudi Arabia's energy minister, seen in this photo from March, said the attack didn't cause any casualties or oil spills. (Mladen Antonov/AFP/Getty Images)

The kingdom's Foreign Ministry condemned the incident in a statement published on the state-run Saudi Press Agency on Monday as a "criminal act" that threatens the "safety of maritime traffic, which reflects negatively on regional and international peace and security."

Al-Falih also said the attack was meant to undermine the "security of oil supplies to consumers all over the world" and emphasized the "joint responsibility of the international community to protect" the safety of maritime navigation and oil tankers.

'No injuries or fatalities'

A statement Sunday from the U.A.E.'s Foreign Ministry put the ships near the country's territorial waters in the Gulf of Oman, east of the port of Fujairah. It said it was investigating "in co-operation with local and international bodies." It said there were "no injuries or fatalities on board the vessels" and "no spillage of harmful chemicals or fuel."

Fujairah's port is about 140 kilometres south of the Strait of Hormuz, the narrow mouth of the Persian Gulf through which a third of all oil at sea is traded. The facility handles oil for bunkering and shipping, as well as general and bulk cargo. It is seen as strategically located, serving shipping routes in the Persian Gulf, the Indian subcontinent and Africa.

A view of the port at Fujairah, which U.A.E.'s state news agency said was operating normally on Monday. (Karim Sahib/AFP/Getty Images)

The incident came after the U.S. Maritime Administration, a division of the U.S. Transportation Department, warned Thursday that Iran could target commercial sea traffic.

"Since early May, there is an increased possibility that Iran and/or its regional proxies could take action against U.S. and partner interests, including oil production infrastructure, after recently threatening to close the Strait of Hormuz," the warning read. "Iran or its proxies could respond by targeting commercial vessels, including oil tankers, or U.S. military vessels in the Red Sea, Bab-el-Mandeb Strait or the Persian Gulf."

Publicly available satellite images of the area taken Sunday showed no smoke or fire.

It remains unclear if the previous warning from the U.S. Maritime Administration is the same perceived threat that prompted the White House to order the USS Abraham Lincoln aircraft carrier strike group and B-52 bombers to the region on May 4.