Pakistan's army chief was visiting Tehran on a mediating mission on Monday as the United States prepared to roll out "the greatest financial offensive ever" targeting Iran's trade partners.
Iran dismissed the U.S. threat, vowing to shut down all oil exports from the Gulf "if the economic war continues".
It issued a new warning to shipping not to pass through the Strait of Hormuz without its permission, listing 45 ships it said had violated its rules and threatening retaliation for any ship-to-ship transfers with them, according to a Reuters report.
U.S. Treasury Secretary Scott Bessent has promised to reveal severe measures on Iran, which has endured near-continuous economic sanctions since the Islamic Revolution of 1979, when he gives a press conference at 1 p.m. EDT (1700 GMT) on Monday.
"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.
Pakistan's army chief Asim Munir, who has built a personal rapport with U.S. President Donald Trump, arrived in Iran on Monday for talks, Iranian media said.
Pakistan said the visit was part of its efforts "to promote regional peace and stability", and a Pakistani source said Munir was expected to meet people close to Iran's supreme leader.