"The rest of the world sees perfectly well that Washington's policy is getting increasingly aggressive and reckless," a Russian Foreign Ministry statement said.
Iran's General Amir Hatami said President Donald Trump's policies have promoted "such traits as selfishness, cruelty and violation of humanitarian principles and international rules."
The president "rejoices over the misery that he thinks he has imposed on ordinary Iranians," Iranian Foreign Minister Javad Zarif wrote on Twitter.
"No one can defeat a nation which pursues resistance," Iranian President Hassan Rouhani said in a televised address, claiming the U.S. and its allies were to blame for the country's poor economy.
Ayatollah Ahmad Khatami pointed out that nuclear bombs are "clearly" against Islamic (or Sharia) law.
"Iraq's constitution does not allow our territory of our country to be used against our neighbors," President Barham Salih said.
"We continue to assess that Iran is not currently undertaking the key nuclear weapons-development activities we judge necessary to produce a nuclear device," Dan Coats said.
"It definitely rattled people," a former senior U.S. administration official told The Wall Street Journal. "People were shocked. It was mind-boggling how cavalier they were about hitting Iran."
"The Americans happily say that these sanctions are unprecedented in history. Yes, they're unprecedented. And the defeat that the Americans will face will be unprecedented, God willing," Supreme Leader Ayatollah Ali Khamenei said.
"The United States is a regime formed on the basis of capitalism, racial discrimination and hegemonism," Major General Mohammad Ali Jafari said.
Economy Minister Bruno Le Maire also stressed that tensions with the U.S. over Iran revealed the need for the EU to "affirm our independence."
"We will not obey such sanctions," Turkish President Recep Tayyip Erdogan said, adding that his government saw Washington's move as "violating the global balance."
"U.S. bullying is backfiring," Foreign Minister Javad Zarif warned earlier on Monday.
"We are in the economic war situation," Iranian President Hassan Rouhani warned as new U.S. sanctions took effect. "We are confronting a bullying enemy."
The U.S. announced on Tuesday new sanctions against Iran Tractor Manufacturing, the largest tractor manufacturer in the Middle East, and Iran's Mobarakeh Steel Company, the largest steel producer in the region.
The World Court at the United Nations has ordered that the United States remove sanctions targeting humanitarian trade, food, medicine and civil aviation.
"Confronting multilateralism is not a sign of strength. Rather, it is a symptom of a weakness of intellect," the Iranian president told the U.N. General Assembly.
"Even if Trump himself is interested in negotiations, the people around him are committed to not letting that happen," said the president of the National Iranian American Council.
Secretary of State Mike Pompeo said on Sunday that President Donald Trump is "happy to talk with folks at any time."
The administration is "pleased with this decision because Iran diverts its economic resources away from its people to spread violence and instability across the globe," the U.S. ambassador to Germany said.
"Currently, Iran and its nuclear program have turned into [one of] the most significant concerns of the EU," Federica Mogherini, the EU representative for foreign affairs and security policy, said Friday.