Iran's President Hassan Rouhani has vowed to skirt US sanctions and keep exporting oil.
"America wanted to cut to zero Iran's oil sales ... but we will continue to sell our oil ... to break sanctions," Rouhani said in comments broadcast on state television, Deutsche Welle reports.
The Trump administration is restoring all sanctions lifted under the 2015 nuclear deal in a bid to pressure Iran.
The sanctions that went into effect on Monday target Iran's oil exports and financial transactions, hitting the country's main source of revenue and hard currency.
"We are in the economic war situation. We are confronting a bullying enemy. We have to stand to win," he said.
"Today the enemy [the United States] is targeting our economy ... the main target of sanctions is our people," Rouhani stated.
The United States has said it wants to reduce Iranian exports to zero barrels and would sanction countries and companies that buy Iran's oil.
Later today US Secretary of State Mike Pompeo is expected to announce eight countries that will be granted temporary waivers from the oil-related sanctions. They are likely to include major buyers of Iranian energy such as Turkey, South Korea, India, Japan and Italy.