President Trump on Monday signed an executive order issuing “hard-hitting” sanctions against Iran, saying it would deny Iran’s supreme leader access to financial instruments.

“Today's action follows a series of aggressive behaviors by the Iranian regime in recent weeks including shooting down a U.S. drone,” Trump said in the Oval Office.

The president said Supreme Leader Ayatollah Ali Khamenei “is responsible for the hostile conduct of the regime.”

He said the order “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.”

Meanwhile, Fox News has confirmed that the U.S. military carried out a cyberattack against Iran last Thursday even as the president nixed plans for airstrikes in response to the recent 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.

Yahoo! News first reported on the retaliatory cyber strike. It came as the White House and Pentagon were also considering military strikes, but President 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. The Trump administration is expected to announce more sanctions on Iran later Monday.