The US threatened Iran with what it called "the greatest financial offensive ever marshalled" as it prepared to roll out economic sanctions on Monday that target Iran's trade partners.
Iran in turn vowed to shut down all oil exports from the Gulf "if the economic war continues."
US Treasury Secretary Scott Bessent will hold a press conference at 1 pm EDT (1700 GMT) on Monday amid promises to reveal even more severe measures on a country that has endured near-continuous economic sanctions since the Islamic Revolution of 1979.
"At dawn begins an economic D-Day — the single greatest financial offensive ever marshalled against an adversary," Bessent wrote in an opinion piece published in the Financial Times on Sunday.
The warring nations have not conducted military strikes against each other for weeks, but they also have not engaged in meaningful talks to end the six-month-old conflict.
Thousands of people have died, most of them in Iran and Lebanon, since the US and Israel began strikes on February 28, degrading much of Iran's conventional military capacity and inflicting economic pain while killing Iranian Supreme Leader Ayatollah Ali Khamenei.
But Iran has preserved enough missile and drone capability to attack its Gulf neighbours and threaten oil tankers in the Strait of Hormuz, bringing shipping in the key waterway to a near standstill and pressuring world fuel prices. The exact state of Iran's nuclear program, which the Americans and Israelis aim to wipe out, remains unknown.
Without detailing specific measures, Bessent signalled the US would target "fearful nations" that practice "appeasement" by engaging with Iran's economy and financial system.
"They would do well to consider the consequences of sustaining it," he wrote in the Financial Times.
Iran has been bracing for the sanctions for days, issuing a series of strongly worded statements hinting at a major military response.
Mohsen Rezaei, secretary of Iran's Supreme National Security Council, on Sunday suggested economic retaliation.
"If the economic war continues, not a single drop of oil will be exported, neither through the Strait of Hormuz nor from anywhere in the Persian Gulf," Rezaei wrote in a social media post. "Iran will regard any country’s participation in or support for America’s economic war against the Iranian people as an act of war."
Bessent previously urged China to cooperate with the US, noting China historically has received half of its oil imports from the Gulf region. A spokesperson for China's embassy in Washington responded with a statement that "sanctions and pressure do not help resolve the problem," while calling for diplomacy.
Iran's economy was already under pressure from international sanctions before the U.S. and Israeli attacks destroyed parts of its infrastructure.
Though Tehran remains outwardly defiant, Iranian officials have warned that further economic punishment could increase hardships, reignite unrest and further erode the Islamic Republic's legitimacy.
Iran entered the war with high inflation, a weakening currency, energy shortages, sanctions and deep structural weaknesses, and must now contend with damaged infrastructure, disrupted trade, lost production and the cost of rebuilding.
In the absence of official face-to-face talks between the US and Iran, which were last conducted in June in Switzerland, other nations including Qatar, Pakistan and Turkey have attempted to promote diplomacy.
Iran said Pakistan's army chief, Asim Munir, will visit Tehran on Monday as part of efforts to restore peace and security in the region, but gave few details.
Pakistan has been mediating in the conflict and a Pakistani government source said Munir would touch on recent developments including the US threat of new sanctions.
US-Israeli strikes on Iran and Israeli attacks on Lebanon during the war have killed thousands and displaced millions. The US reported 18 military personnel killed and more than 750 wounded.
