Thousands have been killed in the war, which quickly drew in Gulf nations and shocked global markets as Iran flexed its ability to curb shipping through the Strait of Hormuz, which carried about a fifth of the world's traded oil before February.
The US and Iran have twice announced ceasefire deals, in April and June, aiming to restore the free flow of shipping through Hormuz on a path towards ending the conflict, but both quickly crumbled even as Israel has largely withdrawn from the fighting.
In a social media message on Wednesday, Trump promised "Economic Warfare and Isolation on an unprecedented scale", although details were scant.
Iran has weathered continuous punishing economic sanctions for nearly 50 years, since the Islamic Revolution of 1979.
"ANY country that allows its financial institutions, businesses, airports, or government entities to provide any type of lifeline to Iran will itself face TREMENDOUS Economic Consequences," Trump wrote.
Iranian Foreign Minister Abbas Araqchi called Trump's comments an attempt to divert American public opinion from domestic financial problems, including record debt and rising interest rates.
He said Washington's insistence on policies he described as failed would bring further failures and alienate Iranians.
"America's economic terrorism threatens the global economy and the national sovereignty of countries around the world," he said on X.
Trump's social media threats and announcements do not always get implemented as written, and he did not say what specific steps the US would take against such a country, which would appear to include US allies that have helped mediate peace talks, but he did not identify any.
On Tuesday, the United Arab Emirates, which hosts a major US military base, said it was suspending all trade activities, commercial exchanges and financial transactions with Iran until further notice.
The step came after the UAE defence ministry said it had detected two missiles launched from Iran that fell into the sea, a report Iran dismissed as baseless.
China buys more than 80 per cent of Iran's shipped oil, 2025 data from analytics firm Kpler shows, but the US risks retaliation should it engage in further economic warfare with China, a major exporter of items such as vital rare earth minerals to the US.
Iran remained open to dialogue with the US but did not confuse negotiations with surrender, Mohammad Mokhber, an adviser to Iran's supreme leader, said on Tuesday, according to the semi-official Fars news agency.
Military pressure and sanctions would not break Iran's resolve, Mokhber added.
On Tuesday, Trump said no talks were taking place with Iran.
A day earlier, Jared Kushner, his son-in-law and special envoy, struck an upbeat tone, saying talks were still under way and "probably more robust" than ever.
Trump wants to seize Iran's stockpile of highly enriched uranium and is seeking a new, more restrictive agreement curbing its nuclear energy and research programs, to replace one from which he unilaterally withdrew the US in 2018.
A signatory to the non-proliferation treaty, Iran has said for decades its nuclear programs are peaceful.
