Oil prices fell Sept. 18 as President Donald Trump tweeted that he asked the U.S. Treasury Department to "substantially increase sanctions" on Iran.
Brent crude fell 1.08% to $63.85 per barrel on the ICE Futures Exchange as at 9:13 a.m. ET.
The statement comes hours after Iran warned the U.S. that any action by Washington against Tehran will result in an immediate reaction, Reuters reported, citing Iran's semiofficial ISNA news agency.
The U.S. had blamed Iran for Sept. 14 drone attacks over Saudi Arabia's Abqaiq and Khurais oil processing plants that led to a reduction in oil production by half.
Relations between the two countries have worsened after the U.S. withdrew a 2015 Iran nuclear deal and reinstated sanctions on the Middle-East nation. News emerged Sept. 11 that Trump was discussing easing sanctions on Iran.
