The United Arab Emirates told United Nations Security Council members on Thursday that attacks on four tankers off its coast on May 12 bore the hallmarks of a “sophisticated and coordinated operation,” most likely by a state actor.

In a document on the briefing to Security Council members, the U.A.E., joined by Norway and Saudi Arabia, did not say who it believed was behind the attacks and did not mention Iran, which the United States has accused of being directly responsible.

The three countries said the attacks required expert navigation of fast boats and trained divers who likely used a high degree of precision to place limpet mines on the vessels under the waterline.

“While investigations are still ongoing, these facts are strong indications that the four attacks were part of a sophisticated and coordinated operation carried out by an actor with significant operational capacity, most likely a state actor,” the three countries said in the document.

U.S. national security adviser John Bolton said on May 29 that the attacks were the work of “naval mines almost certainly from Iran.” Tehran denied the accusations.

A few days earlier in May, at the Pentagon, U.S. Vice-Admiral Michael Gilday accused Iran’s Islamic Revolutionary Guards Corps of being directly responsible for the attacks.

The May 12 attacks targeted two Saudi tankers, an Emirati vessel and a Norwegian tanker, causing no casualties. They occurred off the U.A.E. emirate of Fujairah, which lies just outside the Strait of Hormuz, a vital global oil and gas shipping route.

The strait separates the Gulf Arab states and Iran, which has been embroiled in an escalating war of words with the United States over U.S. sanctions and the U.S. military’s regional presence.