US President Donald Trump ordered “hard-hitting” financial sanctions on Monday on Iran's supreme leader Ayatollah Ali Khamenei, holding him “ultimately responsible” for the Islamic republic's destabilizing activities.

“We will continue to increase pressure on Tehran,” Mr. Trump said as he signed the order in the Oval Office. “Never can Iran have a nuclear weapon.”

“We do not ask for conflict,” he said, adding that depending on Iran's response the sanctions could end tomorrow -- or it “can also be years from now.”