US Secretary of State Mike Pompeo has accused Iran of being behind the reported attacks that damaged two tankers in the Gulf of Oman, offering, however, no concrete evidence in a statement that came hours after Tehran called the incidents "suspicious".
Speaking at a press briefing on Thursday, Pompeo said the presumed attacks were part of a "campaign" of "escalating tension" by Iran and a threat to international peace and security.
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"It is the assessment of the United States that the Islamic Republic of Iran is responsible for the attacks," he said in Washington, DC.
"This is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication," Pompeo added.
He said the US will defend its forces and interests in the region but gave no specifics about any plans and he took no questions.
Speaking earlier in the day, Mohammad Javad Zarif, Iran's foreign minister, said the timing of the reported attacks was "suspicious" since they coincided with a meeting between the country's supreme leader and Japan's Prime Minister Shinzo Abe, who was in Tehran to defuse escalating US-Iran tensions.
Although information of what exactly happened was still scarce, the incidents near the strategically important Strait of Hormuz prompted international alarm and sent oil prices spiralling upward.
The suspected attacks on Thursday left one ship ablaze and both adrift, forcing scores of crew to abandon the tankers.
Washington accused Tehran of being behind a similar attack on May 12 on four tankers in the same area, a vital shipping route through which much of the world's oil passes.
Iran denies the US claim.
Tensions between Iran and the US, along with its allies including Saudi Arabia, have risen since US President Donald Trump withdrew from an international agreement aimed at restricting Tehran's nuclear programme last year.
The US has since re-instated economic sanctions that have had a devastating effect on the Iranian economy. In May, it also rushed an aircraft carrier strike group and other military assets to the Gulf region, citing unspecified threats from Iran.
Washington hopes the increased pressure would force Iran to negotiate a new deal, but Tehran says it will not be "bullied" into new talks with the US.
Instead, it has called for dialogue with countries in the Gulf to ease the "alarming security situation" in the region.
Responding to Thursday's events, the UN Secretary-General Antonio Guterres warned the world could not afford a "major confrontation" in the Gulf.
Condemning the suspected attacks, he said: "Facts must be established and responsibilities clarified."