Iran will announce on Wednesday it is partially withdrawing from the nuclear deal it struck with world powers, a year to the day where US president Donald Trump pulled America out, its state-run news agency reported.
The terms of the withdrawal remain unclear, though the IRNA news agency said President Hassan Rouhani will explain Iran's decision in letters to leaders of Britain, France and Germany.
Iranian foreign minister Mohammed Javad Zarif separately will write to the European Union, the agency said.
The semi-official ISNA news agency reported late Tuesday Mr Zarif had left for Moscow to meet Russian leader Vladimir Putin to discuss bilateral and international issues, without elaborating.
Details of the letters, all to signatories of the 2015 accord, will not be publicly disclosed, it said.
The letters will come as officials in the Islamic Republic previously warned that Iran might increase its uranium enrichment, potentially pulling away from a deal it has sought to salvage for months.
The White House has already announced the deployment of a US aircraft carrier and a bomber wing to the Persian Gulf over unspecified threats from Iran.
The United Nations' nuclear watchdog says Iran has continued to comply with the terms of the 2015 nuclear deal, which saw it limit its enrichment of uranium in exchange for the lifting of economic sanctions.
But American sanctions have wreaked havoc on Iran's already-anaemic economy, while promised help from European partners in the deal have not alleviated the pain.
The US last week stopped issuing waivers for countries importing Iranian crude oil, a crucial source of cash for Iran's government.
It also halted waivers allowing Iran to store excess heavy water in Oman and to swap enriched uranium for raw yellow-cake with Russia.
Mr Trump campaigned on a promise to tear up the deal struck by his predecessor, Barack Obama.
While Mr Trump has sought to dismantle many of Mr Obama's policies, he particularly criticised the Iran nuclear deal for failing to address Tehran's ballistic missile programme and what he described as its malign influence across the rest of the Mideast.