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 2 p.m. EDT (1800 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.

Without detailing specific measures, the US treasury secretary signaled 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," Bessent wrote in the Financial Times.

A defiant Iran

Iran has been bracing for the sanctions for days, issuing a series of strongly worded statements ​suggesting it planned 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."

Iranian Foreign Minister Abbas Araqchi dismissed the threat of new US sanctions ​as a sign of desperation on Sunday and said the expected new measures would fail to defeat Tehran.

"The fact that they (US leaders) have moved on from military operations to bring up the same old plans (economic sanctions) shows that they are desperate," Araqchi said in a post on Telegram.

"We have never been afraid. All of their actions – whether blockades or other military moves they have made – have failed, and this new issue of theirs will fail as well," he said.

Strait of Hormuz nears a standstill

Commercial shipping through the Strait of Hormuz has come to an almost complete standstill amid escalating tensions.

US Central Command said the guided-missile destroyer USS John Finn had redirected 70 commercial vessels, disabled three and boarded two as part of efforts to enforce a US naval blockade of Iran.

Iran's Persian Gulf Strait Authority (PGSA), the body responsible for maritime traffic in the Strait of Hormuz, announced on Sunday in a post on X that vessels violating Iranian protocols for transiting the waterway will face restrictions on their future passages.

The PGSA called on "cargo owners shipping to and from the Persian Gulf" to review the list of "Non-Compliant Vessels" on its website prior to chartering to avoid potential issues.

Meanwhile, Hassan Qashqavi, spokesperson of the Iranian parliament's national security and foreign policy committee, said on Sunday that the committee has approved an article of a draft plan aimed at safeguarding security in the strait, under which the country will charge fees for the services it provides in the waterway, according to the semi-official Fars news agency.

Fewer than ​20 commodity vessels ‌transited the strait ‌at the ‌weekend, shipping data ‌showed on Monday. Four vessels ​crossed the strait on Sunday and 13 on Saturday, initial data from shiptracker Kpler showed. ‌

Uncertain diplomatic future

With official face-to-face talks between the United States and Iran still suspended since they last met in Switzerland in June, other countries, including Qatar and Pakistan, have sought to revive diplomatic efforts.

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 offered 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.

The imminent sanctions have also cast doubt on the prospects for diplomacy, as analysts warned that Washington's pressure campaign could ultimately prove counterproductive.

Li Zixin, a Middle East expert at the China Institute of International Studies, said although Iran would face significant economic pain, the impact would not necessarily translate into political capitulation, in an interview with China Media Group (CMG). 

However, for the US itself, the backlash from sanctions cannot be ignored, Li said, adding that the average price of regular gasoline across the country is currently about 50% higher than before the war, while pressure from imported inflation is also mounting. The resulting increase in household costs will directly affect ordinary Americans.

In Li's view, the harsher the US sanctions become, the more Iran's hardliners are likely to gain the upper hand, further narrowing the space for diplomacy.

This could create a vicious cycle of "sanctions, countermeasures, and further sanctions," Li said, ultimately leaving neither side a winner.

(With input from agencies)