Iran says it has arrested 17 spies who it says were working for the CIA, and sentenced some of them to death.
The intelligence ministry said the suspects had been collecting information in vital sectors – including military and nuclear areas of activity.
US President Donald Trump has dismissed the claims, saying the report is “totally false”.
Tensions have escalated in recent weeks between Washington and Tehran.
President Trump last year abandoned the international nuclear deal with the Iranians, and the US has imposed sweeping economic sanctions on them.
Iran made Monday’s announcement in state media, saying the alleged spies had been arrested in the 12 months to March 2019.
On Friday, Iran seized a UK-flagged tanker in the strategic Strait of Hormuz.
Last month, Iran shot down a US surveillance drone over the strait, accusing it of violating Iranian airspace. But the US military said the drone had been over international waters at the time, and condemned it as an unprovoked attack.
The US has also blamed Iran for two separate attacks on oil tankers in the Gulf of Oman in May and June – allegations Tehran has denied.
Tensions between the two nations have risen sharply since the US tightened sanctions it reimposed on Iran’s oil sector after unilaterally withdrawing from a landmark 2015 nuclear deal.