Secretary of State Mike Pompeo has warned that the United States will exert "relentless" pressure on Iran unless it behaves "like a normal country."
Pompeo made the comments on November 5, hours after Washington imposed sanctions targeting key sectors of Iran's economy following President Donald Trump's decision to withdraw the United States from the 2015 nuclear agreement between Iran and world powers.
Iran urged the United Nations to hold the United States accountable for the sanctions targeting its energy, shipping, shipbuilding, and financial sectors, branding the measures "illegal" and in violation of a Security Council resolution. Officials earlier struck a defiant tone, saying the country would "bypass" the U.S. measures.
Labeled by Trump as the "toughest ever," the sanctions took force early in the day as part of U.S. efforts to ramp up pressure on Tehran to "change its behavior."
Washington says the terms of the nuclear accord, under which sanctions against Iran were lifted in exchange for curbs on the country's nuclear activities, were not strict enough to prevent Iran from developing nuclear weapons. It also accuses Tehran of supporting militant violence in the region and other "malign" activities.
Iranian officials have denied the allegations.
Eight Oil Waivers
The U.S. Treasury Department said the sanctions targeted more than 200 individuals and companies in Iran's shipping and energy sectors -- including the Atomic Energy Organization of Iran and its 23 subsidiaries and associated individuals, as well as 50 Iranian banks and their subsidiaries.
As part of the round of measures, the White House has warned Iran's customers they must reduce their purchases of oil to zero or face U.S. penalties.
However, it has issued 180-day waivers to eight countries -- China, India, South Korea, Turkey, Italy, Greece, Japan, and Taiwan -- that will allow them to continue importing Iranian oil.
Trump said he wanted to impose sanctions on Iran's oil gradually, citing concerns about causing global price spikes.
"We have the toughest sanctions ever imposed, but on oil we want to go a little bit slower," he told reporters, according to Reuters news agency.
Pompeo said the sanctions will accelerate the rapid decline of Iran's international economic activity, saying over 20 countries had already cut their oil intake from Iran, reducing its exports by more than 1 million barrels a day.
"The Iranian regime has a choice: it can either do a 180-degree turn from its outlaw course of action and act like a normal country, or it can see its economy crumble," he told reporters.
Iran's economy has been hit hard, with the country's currency, the rial, plummeting in value, leading to some street protests in Iranian cities.
The country has lost around $2 billion in oil revenue and lost tens of billions of dollars in investment since May, when the United States pulled out of the nuclear agreement, the U.S. special envoy for Iran, Brian Hook, told a conference call.
Meanwhile, Treasury Secretary Steven Mnuchin said that despite granting the waivers, Washington will "aggressively enforce" the sanctions.
Mnuchin said he expected European countries to honor the U.S. sanctions against Iran. But he said there also are certain transactions, including humanitarian, that will be allowed.
In Brussels, the world's biggest interbank-transfer network, SWIFT, announced it was cutting off links with "certain Iranian banks' access," further isolating Iran from the international financial system.
The service said in a statement that it had taken this "regrettable" step "in the interest of the stability and integrity of the wider global financial system."
It did not mention the U.S. sanctions imposed on Iran, but Washington has told SWIFT it could face sanctions if it fails to comply with the restrictions.
Iranian state TV quoted the head of the Central Bank as saying that Tehran had taken necessary banking measures to continue trade with its partners.
"We have been in talks with our trade partners and all the necessary actions have been taken for Iran's interactions to continue," Abdolnassr Hemmati said.
At the United Nations, Iran's ambassador wrote in a letter to Secretary-General Antonio Guterres that the "irresponsible conduct of the United States necessitates a collective response to uphold the rule of law."
The United States "not only blatantly defies the Security Council Resolution 2231" that endorsed the Iran nuclear accord, "but also audaciously coerces other states to violate the resolution," according to Ambassador Gholamali Khoshroo.
"The United Nations and its member-states, in accordance with the charter of the United Nations and the international law, should resist against these wrongful acts and hold the United States accountable for such acts," Khoshroo wrote.
In a speech on state TV, Iranian President Hassan Rohani earlier said the country was facing a "war situation" and vowed that Iran will "proudly bypass" U.S. sanctions to sell oil.
Meanwhile, state television aired footage of air-defense systems that are involved in two-day military maneuvers across northern Iran.
Israeli Prime Minister Benjamin Netanyahu lauded the U.S. sanctions as "historic."
"Today is the day the U.S. under President Trump's leadership imposed extremely harsh sanctions on Iran, the harshest sanctions imposed on Iran since the effort to curb its aggression began," he said.
Israel has been a fierce opponent of the Iran nuclear deal, saying it didn't rein in Iran's regional military activities.
Meanwhile, China's Foreign Ministry said Beijing regretted the U.S. decision and said it stood with the countries that have vowed to keep the Iran nuclear deal alive despite the U.S. withdrawal.
Britain, France, Germany, Russia, and China, which also signed the nuclear deal along with the United States, insist Iran has abided by its commitments and say they are determined to save the agreement.