Iran's President Hassan Rouhani said "will proudly bypass sanctions" by the United States that took effect on Monday targeting the country's oil and financial sectors.
"I announce that we will proudly bypass your illegal, unjust sanctions because it's against international regulations," Rouhani said in a televised speech.
"We are in a situation of economic war, confronting a bullying power. I don't think that in the history of America, someone has entered the White House who is so against law and international conventions," he added.
The latest tranche aim to significantly cut Iran's oil exports -- which have already fallen by around one million barrels a day since May -- and cut it off from international finance.
The United States has given temporary exemptions to eight countries -- including India, Japan and Turkey -- to continue buying oil in a bid to avoid disturbing their economies and global markets.
But US Secretary of State Mike Pompeo vowed to push Iran's oil sales to zero.
"Watch what we do. Watch as we've already taken more crude oil off the market than any time in previous history," he told CBS's "Face the Nation" on Sunday.
Trump's administration says it wants a new deal with Iran that curbs its interventions around the Middle East and missile programme -- demands that have been flatly rejected by Tehran.
"Constantly they are sending us messages saying 'Let's sit and negotiate.' Negotiations for what?" said Rouhani.
"First, you respect the negotiations we already concluded, so that there are grounds for the next negotiations."