The United States is set to unleash what Treasury Secretary Scott Bessent has described as an “economic D-Day” against Iran, threatening to sever the country’s remaining financial and commercial lifelines.

He also warned governments and businesses that continue dealing with Tehran that they too could face the full force of Washington’s economic power.

Bessent, in a Financial Times opinion article published ahead of a White House-backed announcement of new measures, called the coming campaign “the single greatest financial offensive ever marshalled against an adversary” and said Washington’s objective was to isolate Iran economically until its government stood alone.

The Treasury secretary is due to outline the measures at a press conference on Monday at 2 pm, with the administration expected to expand sanctions beyond Iran itself to countries, financial institutions and businesses that provide Tehran with access to international trade, banking and oil markets.

The threatened escalation comes as diplomatic efforts remain stalled and Iran has responded by threatening to further restrict shipping through the Strait of Hormuz — a move that could put additional pressure on global oil supplies and raise the risk of a wider confrontation in the Gulf.

Bessent warns Iran's economic partners

Bessent's message was aimed as much at Iran's trading partners as at Tehran itself.

“Iran’s enablers purchase and transport its petroleum,” Bessent wrote, accusing countries and companies of facilitating oil sales, allowing ship-to-ship fuel transfers and enabling Iranian access to banks while concealing their involvement.

He warned that governments which continue providing Iran with what Washington considers an economic lifeline should expect to share in Tehran's isolation.

“Any remaining tie to Tehran will hasten the economic ostracism of countries and entities,” Bessent said, according to the op-ed.

The warning follows President Donald Trump's threat last week of “tremendous economic consequences” for any country whose financial institutions, businesses, airports or government entities provide support to Iran. Trump described the coming campaign as an “ECONOMIC D-DAY” and said countries continuing to assist Tehran would face unprecedented economic pressure.

Bessent has also made clear that Washington intends to use secondary sanctions aggressively. In remarks last week, he said countries continuing to buy Iranian oil or transfer money to Tehran could face the full enforcement power of the US Treasury.

The administration appears to be betting that pressure on Iran's remaining sources of foreign currency, oil revenue and international financial access can force Tehran back to negotiations.

Bessent has described the strategy as a combination of economic isolation and pressure on Iran's ability to export and generate revenue. He has argued that previous sanctions campaigns show that such pressure can work when financial restrictions are combined with controls on trade and oil flows.

Tehran threatens to shut off Gulf oil

Iran has responded with a threat that could have consequences far beyond its own economy.

Mohsen Rezaei, secretary of Iran's Supreme National Security Council, said on Sunday that if the economic campaign continued, Iran could stop all oil exports from the Gulf.

Iran also issued a fresh warning to shipping companies on Monday, saying vessels that violate what Tehran describes as its transit protocols could face fines, detention or confiscation on future passages.

Iran's Persian Gulf Strait Authority urged cargo owners to consult a list of vessels accused of violating its rules before chartering ships. It said vessels cooperating with blacklisted ships could also be added to the list.

Oil markets brace for sanctions

Oil prices initially moved higher on expectations that tougher US sanctions could further disrupt Iranian exports and shipping through Hormuz, although prices eased on Monday as investors took profits ahead of Washington's announcement.

Brent crude was around $93.16 a barrel in early trading, down $1.23, or 1.3%, while US West Texas Intermediate was around $85.70, down $1.36, or 1.6%.

The retreat did not signal that supply concerns had disappeared. Rather, traders were positioning for greater clarity on the scope of the US measures and their likely impact on Iranian oil exports and Gulf shipping.

Pressure builds inside Iran

The economic confrontation is also exposing differences within Iran's political leadership.

President Masoud Pezeshkian has said Tehran cannot remain at war indefinitely and has defended the agreement reached with Washington in June, despite reported reservations among senior elements of the Iranian leadership. The agreement had been intended to provide a framework for ending hostilities and moving toward negotiations, but subsequent talks have stalled.

That creates a widening gap between calls for continued resistance and the economic realities facing Iran.

The country entered the conflict after years of sanctions, high inflation, a weakened currency and structural economic problems. The war has added damage to infrastructure, disrupted trade and production and increased the cost of reconstruction.

Washington's calculation is that the additional pressure will eventually make continued confrontation too costly for Tehran.

But Iran has spent decades operating under sanctions and has developed extensive networks for moving oil, accessing foreign currency and maintaining trade with sympathetic or commercially motivated partners.

That makes the effectiveness of the new campaign dependent not only on the severity of US sanctions but also on Washington's ability to persuade or force third countries to comply.

China and other trading partners in focus

China is likely to be particularly important to the success of the campaign because of its role in the global oil trade and its economic relationship with Iran.

Bessent has urged Beijing to cooperate with Washington, while China has pushed back against the sanctions strategy, arguing that pressure and sanctions do not resolve the underlying dispute.

Other countries and financial institutions that continue to facilitate Iranian oil sales could also come under scrutiny if the Trump administration follows through on its threat to impose secondary sanctions.

The resulting confrontation could therefore extend well beyond the US and Iran, creating new pressure on banks, shipping companies, insurers, traders and oil buyers operating across the Gulf and Asia.

Pakistan pushes for diplomacy

Against this backdrop, Pakistan's army chief, Field Marshal Asim Munir, is due to visit Tehran on Monday in an effort to advance regional peace and revive stalled US-Iran diplomacy.