Four ships, including two Saudi oil tankers, were damaged in as yet unexplained "sabotage attacks" at a time of Gulf tensions triggered by a bitter US-Iran standoff.
Amid rising fears of a conflict, here is what we know about Sunday's attacks off the Emirati coast.
A United Arab Emirates government official said the Saudi oil tankers Al-Marzoqah and Amjad were attacked off the emirate of Fujairah along with the Norwegian tanker Andrea Victory and a UAE ship, the A. Michel.
The Andrea Victory's managers, Thome Group, said the tanker had a hole in the hull area "after being struck by an unknown object on the waterline".
The crew were unharmed and the ship was not in any danger of sinking.
Saudi Energy Minister Khalid al-Falih said the kingdom's two tankers suffered "significant damage" but there were no casualties or any oil spill.
Neither Saudi Arabia nor the UAE, both close allies of the United States, have yet given details on the exact nature of the attacks.
Adding to the tensions, Saudi Arabia said Tuesday that two pumping stations on a major east-west oil pipeline had been hit in a drones' attack. Yemen's Iran-backed Huthi rebels said earlier they had hit several vital Saudi targets.
Who was behind ship attacks?
There has been no claim of responsibility, and neither Riyadh nor Abu Dhabi has pointed a finger of blame.
The UAE minister of state for foreign affairs, Anwar Gargash, has said the Emirates would probe the "deliberate sabotage" of the ships.
Asked whether Washington believed Tehran played a role, Brian Hook, the US special envoy for Iran, declined to comment, saying only US authorities would be assisting the investigation at the UAE's request.
What has been the reaction?
Saudi Arabia, Iran's regional arch-rival, condemned "the acts of sabotage which targeted commercial and civilian vessels", a foreign ministry source said.
"This criminal act constitutes a serious threat to the security and safety of maritime navigation and adversely impacts regional and international peace and security," the source added.
Without accusing Tehran, US President Donald Trump warned that Iran would "suffer greatly" if it does "anything".
"I'm hearing little stories about Iran," Trump said at the White House. "If they do anything, it would be a very bad mistake. If they do anything they will suffer greatly."
Britain warned of the danger of conflict erupting "by accident" in the Gulf, as Tehran distanced itself and called for an investigation into the "alarming and regrettable" attacks.
The United Nations urged restraint from all sides.
"We are concerned about the heightened tensions in the region. We call upon all concerned parties to exercise restraint for the sake of regional peace," said UN spokesman Farhan Haq.
What is significance of location?
Fujairah port is the only Emirati terminal located on the Arabian Sea coast, bypassing the Strait of Hormuz, through which most Gulf oil exports pass.
Iran has repeatedly threatened to close the strait in case of a military confrontation with the United States.
According to the Energy Information Administration (EIA), 35 percent of the world's seaborne oil transits passes through the Strait of Hormuz.
Oil transit was disrupted in 1984 during the Iran-Iraq war (1980-1988) when more than 500 vessels were destroyed or damaged in a "Tanker War".
In 1988, an Iran Air flight from Tehran to Dubai was shot down by missiles fired from a US Navy cruiser patrolling the strait. All 290 people on board were killed.
The crew of the USS Vincennes said they mistook the Airbus for an Iranian fighter.
What are the stakes?
Washington's dispatch to the region of the USS Abraham Lincoln carrier group, an amphibious assault ship, a Patriot missile battery and B-52 bombers has sparked fears of possible military clashes.
The increasing tensions come after Tehran said last week it has stopped respecting limits on its nuclear activities agreed under a 2015 deal that has since been abandoned by Washington.
US national security advisor John Bolton has said Washington's military buildup was "a clear and unmistakable message to the Iranian regime that any attack on United States interests or on those of our allies will be met with unrelenting force".