Iran says it has captured 17 spies working for the CIA and sentenced some of them to death, an announcement US President Donald Trump dismissed as "totally false".
The Iranian announcement came hours before Britain was due to unveil its response to Iran's seizure of a British oil tanker.
The move has escalated a three-month confrontation that nearly drew the US and Iran into a shooting war.
"The Report of Iran capturing CIA spies is totally false. Zero truth. Just more lies and propaganda (like their shot-down drone) put out by a Religious Regime that is Badly Failing and has no idea what to do. Their Economy is dead, and will get much worse. Iran is a total mess!" Trump tweeted on Monday.
Iranian state television published images it said showed CIA officers who had been in touch with the suspected spies.
The Ministry of Intelligence said the 17 spies had been arrested in the 12 months to March 2019. Some had been sentenced to death, according to another report.
Such announcements are not unusual in Iran and are often made for domestic consumption. But the timing suggested a hardening of the Iranian position as the Gulf crisis escalates.
British ministers are expected to use a speech to parliament later on Monday to unveil plans to respond to Friday's capture of the Stena Impero tanker by Iranian commandoes in the Strait of Hormuz.
Regional experts say London has few good options to exert leverage over Iran at a time when Washington has already imposed the maximum possible economic sanctions, banning Iran's global oil exports.
But the incident may prompt Britain and other countries to be more forthcoming as Washington has asked its reluctant allies to provide more ships to help secure the Gulf.
Asked on Fox News about any possible US role over the seized tanker, US Secretary of State Mike Pompeo was unequivocal.
"The responsibility ... falls to the United Kingdom to take care of their ships," he said.