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.
In recent weeks, the two countries have come close to military conflict in the Gulf.
What do we know about the ‘spying case’?
Iran says the spies who are alleged to work for the US Central Intelligence Agency were arrested over a 12 month period up until March this year.
The 17 were all Iranians working in “sensitive centres” in military and nuclear facilities and the private sector who had acted independently of each other, a top Iranian intelligence official told reporters.
He did not say how many had been sentenced to death or when the sentences were handed down.
“Sentences for these spies have been issued, some of whom are to be put to death as ‘corruptors on earth’ [a charge punishable by death under Islamic rules in Iran],” the Iranian Students News Agency (ISNA) reported, quoting the head of the intelligence ministry’s espionage department as saying.
On Sunday, Intelligence Minister Mahmoud Alavi announced that a documentary on the arrests of “US-linked spies” was due to be aired on Iranian TV the next day.
“We identified American and foreign intelligence spies in various sensitive centres and we handed them over to the judiciary. Our people will be informed about the details in a documentary that will be aired tomorrow [22 July],” Mr Alavi told state TV, without offering further details of when the arrests took place.
Later on Sunday, the internationally-facing, English-language Press TV aired a documentary on an operation that resulted in the identification and arrest of what it said was a “CIA network” operating inside the country.
Last month, Iran said it had dismantled a network linked to the CIA, but it is unclear whether Monday’s announcement is related to the same case.
What’s the background to the current crisis?
Tensions between the US, the UK and Iran remain high, after a series of incidents in the world’s key shipping area, the Strait of Hormuz.
On Friday, Iran seized a UK-flagged oil tanker in the strategic waters.
Tehran had previously warned it would retaliate after the UK captured an Iranian tanker off the coast of Gibraltar earlier this month.
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.