President Trump on Monday announced additional U.S. sanctions against Iran targeting the country's supreme leader and other high-ranking officials.

Trump signed an executive order in the Oval Office that he said will deny the supreme leader and others access to financial instruments. Trump had previously signaled he would levy additional sanctions on Iran amid simmering tensions between Washington and Tehran.

"The supreme leader of Iran is one who ultimately is responsible for the hostile conduct of the regime," Trump said. "He’s respected within his country. His office oversees the regime's most brutal instruments."

He called the new sanctions a "strong and proportionate response to Iran’s increasingly provocative actions." Trump noted the recent downing of an unmanned U.S. surveillance drone near Iran, and attacks on oil tankers in the Strait of Hormuz.

Treasury Secretary Steven Mnuchin, briefing reporters a short time later, clarified that some of the sanctions announced Monday were already in the works, while others were "the result of recent activities." He declined to specify which sanctions fell into which category.

The sanctions will apply to Supreme Leader Ayatollah Ali Khamenei and his office, military officer Alireza Tangsiri for his threat to close the Strait of Hormuz, two individuals tied to the downing of the U.S. drone and five Iranian naval district leaders.

Mnuchin said the administration will also designate Iranian Foreign Minister Mohammad Javad Zarif later in the week.

Iran last week shot down the U.S. drone, which Tehran said violated its airspace. U.S. officials have maintained the aircraft was in international airspace, calling the action "unprovoked."

Iran said Tuesday new US sanctions against its leaders marked the end of diplomacy with Washington, after President Donald Trump threatened the country with "obliteration".

Iran's Foreign Ministry spokesman Abbas Mousavi said Tuesday that "imposing fruitless sanctions against Iran's supreme leader and the commander of Iran's diplomacy is the permanent closure of the path to diplomacy with Trump's desperate government."

"Trump's government is destroying all established international mechanisms for keeping global peace and security," he said in a tweet.

Trump said Friday that he called off a retaliatory strike after learning it would result in 150 Iranian deaths. The president has alternated in recent days between warning of the potential for U.S. military action and emphasizing his preference to negotiate with Iranian leaders.

"We do not seek conflict with Iran or any other country,” Trump said on Monday. "I look forward to discussing whatever I have to discuss with anybody that wants to speak. In the meantime, who knows what's going to happen. I can only tell you we cannot ever let Iran have a nuclear weapon."

Trump added that the sanctions were "going to happen anyway" when asked whether they were a response to the downing of the drone.

"I think a lot of restraint has been shown by us, a lot of restraint, and that doesn’t mean we’re going to show it in the future,” the president said. "But I felt I wanted to give this a chance, give it a good chance. Because I think Iran has potentially a phenomenal future."

Monday's sanctions targeting Iran's leaders marked a shift in strategy for the Trump administration.

The Trump administration has imposed numerous sanctions against Iran since the president withdrew from the Obama-era Iran nuclear deal last year. Previous penalties have hit the Iranian oil and metals sectors, as well as the Iranian Revolutionary Guard Corps in an effort to hamper the country's economy.

"Our issue is not with the people of Iran," Mnuchin said. "We are not looking at creating issues for the people of Iran. Having said that, we have sanctions against bad behavior, and there's no question that locking this money up worked last time, and there's no question locking the money up works now.”