US National Security Adviser John Bolton on Tuesday claimed that recent attacks on four oil tankers in the Gulf of  Oman involved “naval mines almost certainly from Iran.” Bolton did not provide evidence to support his claim.

The UAE described the incident on 12 May as a “sabotage attack” that targeted two Saudi-Arabian vessels, one from Norway and one from the UAE. Iran has denied responsibility for the attack.

Bolton’s allegations come at a time of heightened tensions between the US and Iran. Both international analysts and Iranian officials have identified John Bolton as the driving force behind America’s tougher stance, which is fueling global worries about a possible military conflict between the two countries.

Read more: Tankers almost certainly damaged by Iranian naval mines, US says