US National Security Adviser John Bolton said on Wednesday that attacks on oil tankers off the coast of the United Arab Emirates this month were the work of “naval mines almost certainly from Iran.” The UAE has not yet blamed anyone for the sabotage of four vessels, including two Saudi tankers, which was followed two days later by drone strikes on oil pumping stations in Saudi Arabia. Riyadh accused Tehran of ordering the strikes, which were claimed by Yemen’s Iran-aligned Houthi movement. Iran has denied involvement in either attack. “I think it is clear these (tanker attacks) were naval mines almost certainly from Iran,” Bolton told reporters in Abu Dhabi but declined to comment on the specifics of the investigation in which the United States is taking part. He said the tanker attacks were connected to the strike on oil pumping stations on Saudi Arabia’s East-West pipeline and a rocket attack on the Green Zone in the Iraqi capital Baghdad. Iran’s foreign ministry spokesman meanwhile dismissed the remarks made by Bolton that Iranian naval mines likely used in the UAE tanker attacks, Fars News said. Bolton also said there had been an unsuccessful attack on Saudi Arabia’s Red Sea port of Yanbu a couple of days before the operation off the coast of the UAE. Saudi officials were not immediately available to for comment. “We take all of this very seriously,” he said. “These attacks were unfortunately consistent with the very serious threat information that we had been obtaining. It is one reason we increased our deterrent capability in the region.” Tensions between the United States and Iran have escalated since US President Donald Trump withdrew from a pact, signed with other major powers, designed to curb Tehran’s nuclear activities.