"I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!" Trump said via Twitter.
The sanctions appear to be part of Trump's response to Iran's alleged involvement in the weekend attacks on Saudi oil facilities.
The statement comes after Saudi Arabia’s Energy Minister Prince Abdulaziz bin Salman said on 17 September that oil production had returned to the previous levels.
Prior to that, Saudi Aramco had to close two of its compounds, the Abqaiq and Khurais facilities, after they were hit by drones and caught fire. The incident led to a cut in oil production totalling 5.7 million barrels per day - about half of Saudi Arabia’s daily oil output. The closure of the facilities triggered a surge in oil prices worldwide.
US officials, including Trump, have accused Iran of being responsible for 14 September's attacks. Tehran has strongly denied the accusations. The Houthi movement, against which Riyadh and its allies have waged a four-year war, claimed responsibility for the attack and promised more strikes.
Tensions between Iran and the United States have escalated recently. Relations substantially worsened last year after US President Donald Trump withdrew his country from the 2015 nuclear agreement with Iran and began re-imposing sanctions. In May and June, Washington blamed Iran for a series of security incidents, including attacks on tankers in the Gulf of Oman, and built up its military presence in the Middle East. Iran has rejected the allegations and accused the United States of trying to find a pretext for conflict.