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(CNN) —  

US national security adviser John Bolton blamed Iran on Wednesday for the attack on four oil tankers at a port in United Arab Emirates this month, without offering evidence that Tehran was responsible.

“I think it is clear these (attacks) were naval mines almost certainly from Iran,” Bolton told reporters during a visit to Abu Dhabi on Wednesday.

Iran later rejected Bolton’s claim as “ridiculous” that it was responsible for the May 12 attack on the four oil vessels near the Emirati port of Fujairah.

Bolton also said Wednesday that the additional 1,500 US troops sent to the Middle East region are to “act as a deterrent” against Iranian threats. The Trump administration has also sent a carrier and bomber task force to the Persian Gulf.

“The point is to make it very clear to Iran and its surrogates that these kinds of actions risk a very strong response from the United States,” he said.

Following Bolton’s visit to the UAE, the White House released a joint statement from the two countries announcing a new Defense Cooperation Agreement.

The agreement will lead to “closer collaboration on defense and security matters and supporting efforts by both nations to maintain security in the Gulf region,” according to a press release.

The top US general also spoke about Iran on Wednesday, saying he had seen intelligence earlier this month on threats posed by Tehran that were “qualitatively different” from what he had been seen emanating from there previously.

“What was new was a pattern of threat streams that extended from Yemen, so threats emanating from Yemen, threats in the Gulf and threats in Iraq,” Joint Chiefs Chairman Gen. Joseph Dunford said at the Brookings Institution in Washington.

Of the four commercial ships targeted in the May 12 attack, one was flying a UAE flag, two were tankers owned by Saudi Arabia, and the fourth was a Norwegian tanker. The UAE, which described the attack as a “sabotage,” did not elaborate on how the ships were damaged or who might be responsible. Iran denounced the attack and denied involvement.

The Pentagon recently said intelligence reporting showed that Iran’s Revolutionary Guard Corps was responsible for the attack on the oil tankers – but did not provide any evidence of Iran’s role in the attack.

“I can’t reveal the sources of that reporting except to say with very high confidence we tie the Iranians to those,” Vice Adm. Michael Gilday, director of the Joint Staff, told reporters on Friday.

Iran dismissed the accusation from the United States on Wednesday that it was responsible for the May 12 attack, and blamed Bolton, the Saudi Crown Prince and Abu Dhabi’s Crown Prince for “dragging” President Donald Trump “into war with Iran,” according to state-run media.

Bolton had previously advocated for regime change in Iran before he joined the Trump administration.

Trump said Monday that he’s not looking for regime change in Tehran, but merely wants to avoid Iran developing or acquiring nuclear weapons.

But acting Secretary of Defense Patrick Shanahan noted Wednesday that the threat from Iran remains.

“I don’t see a change in any behavior. I think the situation is, still remains, tense. It’s a high-threat environment but I haven’t seen it change in the last few days,” he said.

CNN’s Nada AlTaher, Barbara Starr and Ryan Browne contributed to this report.