U.S. National Security Adviser John Bolton said Tuesday that Iran was “almost certainly” responsible for attacks earlier this month on oil tankers off the coast of the United Arab Emirates.

Speaking to reporters during a visit to the Abu Dhabi for what he called talks on “important and timely regional security matters,” Bolton said the tankers were hit by naval mines.

Iran previously denied responsibility for the attacks.

Tensions between Iran and the United States have escalated in recent weeks since President Donald Trump announced his decision to try to cut Iran’s oil exports to zero to pressure the Iranian government to alter its behavior in the Middle East, and his move to boost the U.S. military presence in the Gulf in response to what he said were Iranian threats.

Iran’s Foreign Minister Mohammad Javad Zarif has accused Bolton and others of engaging in “warmongering” and said the Trump administration with its re-imposition of sanctions against Iran is conducting “economic terrorism.”