Khamenei
© Official Khamenei website/Handout via REUTERS
President Trump struck back Monday at Iran by issuing "hard-hitting" financial sanctions against Supreme Leader Ayatollah Ali Khamenei and his associates.

"Today's action follows a series of aggressive behaviors by the Iranian regime in recent weeks including shooting down a U.S. drone," the president said in the Oval Office, calling Khamenei "responsible for the hostile conduct of the regime."

Trump said the sanctions "will deny the supreme leader and the supreme leader's office and those closely affiliated with him and the office access to key financial resources and support." Speaking to reporters in the White House briefing room Monday, Treasury Secretary Steven Mnuchin said the sanctions "lock up literally billions of dollars more of assets."

"Along with that action today, we are also announcing specific actions targeting those responsible for recent activities," Mnuchin said, adding that the president has instructed him to sanction Iran's Foreign Minister Javad Zarif "later this week."

The Treasury Department, in a news release, said "any foreign financial institution that knowingly facilitates a significant financial transaction for entities designated under this Executive Order could be cut off from the U.S. financial system."

Amid the newly announced sanctions, Fox News has confirmed that the U.S. military also carried out a cyberattack against Iran last Thursday even as the president nixed plans for airstrikes in response to the downing of an American drone.

Sources said U.S. Cyber Command launched the cyberattack targeting the Iranian intelligence and radar installations used to down the U.S. Navy drone last week.

Fox News has learned Iran shut off some of its military radar sites around the time the U.S. was poised to launch retaliatory strikes. It's not clear if those radar sites were turned off by the cyberattack or if Iran shut them off deliberately in anticipation of this.

Treasury Secretary Steve Mnuchin appears before the White House press corps to lay out the newest sanctions on Iran and take questions.

Yahoo! News first reported on the retaliatory cyber strike. It came as the White House and Pentagon were also considering military strikes, but Trump revealed last Friday that he called them off after learning up to 150 Iranians could be killed.

"I stopped it, not ... proportionate to shooting down an unmanned drone," he tweeted at the time.

The drone shoot-down was only the latest flare-up in the region tied to Iran.

In recent weeks, six oil tankers have been attacked in the Gulf of Oman, which the U.S. and its Gulf allies blame on Iran.