But the exemptions granted to Iran's biggest oil clients - China, India, South Korea, Japan, Italy, Greece, Taiwan and Turkey - allow them to import at least some oil for another 180 days and could mean exports start to rise after November.
"The decision by the U.S".
The U.S. restored sanctions on Iran's oil industry this week that had been lifted under the 2015 nuclear accord, which President Donald Trump withdrew from in May.
Democratic Senator Tom Udall, who is also a member of the Senate Foreign Relations Committee, said, "Led by people like John Bolton, who believe that we should bomb Iran, I fear the Trump administration is marching toward another catastrophic war in the Middle East". Crude exports contribute one-third of Iran's government revenues.
Mr Pompeo said more than 20 countries have already cut their oil imports from Iran, reducing purchases by more than 1 million barrels per day.
In October, Iran's crude exports were estimated at 1.82 million bpd by Kpler and 1.5 million bpd by another firm that monitors Iranian shipments.
"While the Iranian sanctions should still be viewed as a latent bullish consideration capable of limiting much additional price slippage, it would appear that the Iranian factor alone will not be capable of spurring higher prices without major assistance from a renewed strengthening in the equities, sustainable weakening in the US dollar or a significant cut back in OPEC production", Jim Ritterbusch, president of Ritterbusch & Associates, said in a note.
He also said the reimposed USA sanctions were part of a psychological war launched by Washington against Tehran, adding that "America's economic pressure on Iran is futile".
They include more corporate entities and individuals compared with sanctions imposed on Iran during the tenure of President Barack Obama, Trump's predecessor.
At the same time, the trade dispute between the United States and China is threatening growth in the world's two biggest economies.
House Democratic Whip Steny H Hoyer said, "Rather than achieving its stated goal of eliminating Iran's oil exports altogether, the administration has issued "exemptions" for major Iranian oil importers, allowing Iran to earn billions of dollars from oil sale".
That would be about half the daily average China has been importing from Iran since January 2016, trade data showed.
"Today the enemy (the United States) is targeting our economy ... the main target of sanctions is our people", Rouhani said.
The US on Monday imposed "the toughest ever" sanctions on a defiant Iran aimed at altering the Iranian regime's "behaviour".
International Brent crude oil futures were down 21 cents, or 0.3 per cent, at $72.96 a barrel. The session low was $61.31 a barrel, the weakest since March 16.
Japan says it is pleased to be temporarily spared from penalties as the USA resumes sanctions on Iran.
On the supply side, U.S. crude oil production is expected to average 12.06 million barrels per day (bpd) in 2019, passing the 12 million bpd milestone sooner than expected on surging domestic shale output, the U.S. Energy Information Administration said on Tuesday.