Secretary of State Mike Pompeo on Wednesday called the weekend drone strikes on a Saudi oil facility an "act of war" and an "Iranian attack."
Pompeo made the comments from Jeddah, Saudi Arabia.
President Trump ramped up the pressure on Iran after an attack on a key Saudi oil facility, telling the Treasury Department to "substantially increase" sanctions on Iran.
Trump announced the directive on Twitter.
"I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!" he tweeted Wednesday.
The weekend attacks on the oil field, conducted via drone, caused oil prices to surge on Monday and raised tensions in the Middle East. The U.S. has blamed Iran for the attack, while Saudi Arabia said Wednesday it was "unquestionably sponsored" by Iran.
Iran told the U.S. that it would respond "immediately" to "any action" against Iran, according to a state-run news agency.
"If any action takes place against Iran, the action will be faced by Iran's answer immediately," Iran wrote in a note, according to its news agency.
Iran has denied responsibility for the attack, which knocked out around 5 percent of the world's oil supply. Saudi Arabia said Tuesday that its oil production would be back to normal levels by the end of September.
"The damage has been contained," Energy Minister Prince Abdulaziz bin Salman said.
The U.S. reimposed sanctions on Iran in November 2018 that had been lifted under the Iran nuclear agreement, which Trump pulled the U.S. out of in May 2018. Trump imposed new sanctions on Iran in June 2019.
Secretary of State Mike Pompeo said earlier this month that the Iran sanctions could cut its gross domestic product by 12 percent.
Saudi officials said in a press conference Wednesday they recovered Iranian cruise missiles and drones, and that they are sure that the attack came from the north.
The Iranian news agency also reported Iran's president and the foreign minister may not be able to attend next week's high-level meetings at the United Nations as the U.S. has reportedly yet to issue them visas. Such a meeting was previously seen as a chance to cool growing tensions between Iranian President Hassan Rouhani and Trump.
Pompeo is en route to Saudi Arabia to assess the aftermath of Saturday's attack.
Trump also used Twitter to remind the world the U.S. is no longer energy-dependent.
"So nice that our Country is now Energy Independent," Trump tweeted Wednesday. "The USA is in better shape than ever before."
The Associated Press contributed to this report.