Brian Hook, Special Representative for Iran and Senior Policy Advisor to the Secretary of State, at the United Against Nuclear Iran (UANI) 2018 Iran Summit in New York City. (Photo by Michael Brochstein/SOPA Images/LightRocket via Getty Images) Michael Brochstein/SOPA Images | LightRocket | Getty Images

The September attack on Saudi Aramco's facilities that temporarily shut down half of the kingdom's oil production represented an act of war by the Iranian state, U.S. special representative for Iran Brian Hook told CNBC on Saturday. "Because of the Iran nuclear deal we've been accumulating risk of a regional conflict — and what Iran did to Saudi Arabia on September 14 was an act of war," Hook told CNBC's Hadley Gamble during the Doha forum in Qatar. Iran's government has stringently denied involvement in the drone and missile attack, considered to be the most significant assault on oil infrastructure in history.

The asymmetric advantage that any terrorist regime enjoys, it's impossible to eliminate. Brian Hook U.S. special representative for Iran

Riyadh, alongside Washington and several other Western allies, has accused Iran of involvement in the attack. But it has not directly accused the Islamic Republic of carrying out an act of war — something seen as an attempt to avoid greater escalation. "To launch an attack from your territory, if that is the case ... this would be considered an act of war," Saudi Minister of State for Foreign Affairs Adel al-Jubeir said in late September. But the kingdom maintains it is currently seeking a peaceful resolution. In response to the suggestion that nothing has been done to Iran as a result of its suspected attack, Hook emphasized the role of diplomacy and the United Nations — something that the Donald Trump administration has been accused of ignoring. "We have, but Iran is more diplomatically isolated as a consequence of it," Hook said. "We still see a role for the UN Security Council to play, and now that Saudi has concluded its investigation, we hope they will do (something) with the UN Security Council." The Trump administration has sent some 14,000 additional U.S. troops to the Gulf region since last spring and has pledged to continue supporting and enhancing Saudi Arabia's air defenses.

Terrorism 'impossible to eliminate'