Attacks on four tankers off the Emirati port city of Fujairah this month were caused by “naval mines almost certainly from Iran”, US National Security Adviser John Bolton said on Wednesday.
The four ships were hit by saboteurs on May 12, prompting condemnation from Gulf countries and an international investigation.
The UAE said it would wait for the investigation results before apportioning blame for the attack. US experts are part of that investigation.
“It’s clear that Iran is behind the Fujairah attack," Mr Bolton said at a US embassy briefing in Abu Dhabi. "Who else would you think would be doing it? Someone from Nepal?
“But I think there’s no doubt in anybody’s minds in Washington. We know who is responsible and I think it’s important that the leadership in Iran know that we know."
Iranian Foreign Ministry spokesman Abbas Mousavi said Mr Bolton's claims were laughable.
Mr Bolton said the US had provided assistance to the investigation, but he declined to discuss specifics saying they would be for the companies and countries involved to disclose.
He revealed there was a failed attack recently on the Saudi oil-shipping port of Yanbu, which would represent a fourth attempted attack linked to Iran and its proxies since the start of May.
It is the first time that any official has said Yanbu was a target in recent months.
Iran-backed Houthi rebels also launched a drone attack on an oil pipeline in Saudi Arabia and Iran-backed militias were blamed for a rocket attack near the US embassy in Baghdad on May 19.
“Since those three, there has not been any further action by Iran or their surrogates in the region that we’re aware of, but we remain very concerned about this and want to be as watchful as we can,” Mr Bolton said.
He said the US was being “prudent and responsible” in gathering evidence around the three attacks and that the extra troops announced for the region were a deterrence against further action.
“We are consulting and co-operating more closely with allies in the region and that’s one of the reasons I’m here today, to discuss what to do next.”
He believed the UAE and Saudi Arabia were on “the same page” about the risk of Iran having nuclear weapons.
Mr Bolton arrived in Abu Dhabi late on Tuesday for talks.
On Wednesday it was revealed that the Defence Co-operation Agreement, signed this year between the UAE and US, will be brought into force.
The agreement will enhance military co-ordination between the two nations, further advancing a robust military, political and economic partnership at a critical time.
It will see closer collaboration on defence and security matters to maintain security in the Gulf region.
Mr Bolton discussed the regional situation and battling terrorism with Sheikh Mohamed bin Zayed, Crown Prince of Abu Dhabi and Deputy Supreme Commander of the Armed Forces.
"The Abu Dhabi Crown Prince and the US National Security Adviser also took stock of the latest regional and global issues of mutual interest, and consequences of unfolding developments in the region," the state news agency Wam reported.
Mr Bolton expressed concern about the threats posed by Iranian proxies in Iraq to the US embassy in Baghdad.
“We’re very concerned by the Quds Force and Qassem Suleimani using Shiite militia groups and others in Iraq as an indirect way to attack our embassy in Baghdad, consulate in Erbil, and various bases around the country,” he said.
Mr Suleimani is the commander of the Quds Force, the elite foreign operations arm of Iran’s Islamic Revolutionary Guard Corps.
Mr Bolton said that if there were “further indirect attacks” by Iranian proxies, Washington would hold the Quds Force responsible.
“We know who sends these rockets into Iraq, we know who trains the Shiite militia groups, we know who finances the Shiite militia groups and we know who, to a large extent, directs their activities," he said.
"We will hold them responsible if they make the mistake of going further."
Before leaving Japan, US President Donald Trump on Tuesday said he was not looking for regime change in Iran but was concerned about Iran having the means to build a nuclear missile.
Mr Trump said he believed he could make a deal with Iran. In response, Iranian President Hassan Rouhani on Wednesday said that the US would need to lift sanctions for any discussion to take place.
“Whenever they lift the unjust sanctions and fulfil their commitments and return to the negotiations table, which they left, the door is not closed," Mr Rouhani said.
Regional tension has sharply increased since Donald Trump's administration reimposed sanctions against Iran last year after it pulled out of the 2015 nuclear accord between Tehran and global powers.
This month, Iran suspended two of its commitments under the nuclear deal on the anniversary of the US withdrawal.
Mr Bolton said there was no reason for Iran to breach the terms of the agreement “unless it is to reduce the breakout time to nuclear weapons”.
He said that if Iran exceeded any of the deal's limits it would result in "very serious" repercussions, but he declined to elaborate.
“First thing you don’t do is tell them in advance what you’re thinking of doing,” Mr Bolton said.
Regional summits are planned on Thursday and Friday in Makkah as Saudi Arabia seeks Arab consensus on Iran to further isolate it.
Mr Bolton said Mr Trump would be pleased to negotiate with Tehran.
“The president has made it very clear that he’s very happy to negotiate with Iran if they want to talk about it, as he’s spoken to Kim Jong-un of North Korea,” he said.
Mr Bolton has led an increasingly harsh US policy on Tehran but insisted that all actions were ordered by the president.
“I'm the national security adviser, not the national security decision maker, so it's really up to him to make those decisions,” he said.
Mr Bolton said the recent attacks on tankers, Saudi oil-pumping stations and Baghdad’s diplomatic Green Zone were “manifestations of concern” about Iran.
Several prominent Iranian-backed militias, part of Iraq’s Popular Mobilisation Forces established to fight ISIS after 2014, have criticised the rocket attack on the Green Zone, saying it was not the right time for such action.
Tehran has also tried to distance itself from the bombings but on Sunday Foreign Minister Javad Zarif said his country would defend itself against any military or economic aggression.
Mr Mousavi, said on Tuesday that unless the US offered sanctions relief, Tehran “will, with severity, take the next steps”.
The US administration has ordered all non-essential diplomatic staff to leave Iraq, saying there were threats from Iranian-backed Iraqi armed groups.
It also sent an aircraft carrier and heavy B-52 bombers to the region.
Mr Bolton said the measures were to keep the Patriot missile defence system in the region, to defend American troops and engineers and to protect Washington’s bases in the Gulf.
He said the US was taking “steps that we think will demonstrate our forces are not vulnerable, our civilian personnel and embassies and consulates are not vulnerable, certainly not as vulnerable as Iran may have thought”.
On Friday, Mr Trump announced that 1,500 soldiers would be sent to the Middle East.
The same day, the White House also used Iranian threats to speed up the sale of $8.1 billion (Dh29.1bn) of arms to Saudi Arabia, the UAE and Jordan.
Mr Bolton said Mr Trump withdrew from the Iranian nuclear agreement because it did not constrain Tehran.
He said that the issue of Iranian nuclear ambitions were “right at the top of everybody’s agenda”.