Iran claims it has captured 17 "CIA spies" and some have been sentenced to death as the crisis with the West took a dramatic turn.
An intelligence ministry official claims CIA operatives infiltrated "sensitive" and "vital" sectors of Iran - including economic, nuclear, military and cyber sectors - to steal secrets.
Some of those involved in the spy ring were sentenced to execution for being "corrupters", the official added.
Iranian media then published names and photos claiming they showed "CIA officers" who were in charge of spies in Iran. Two photos showed men with their partners and young children.
The US has not yet responded to Tehran's claims.
Iran announced the arrests amid its spiralling crisis with the West and escalating tensions in the Gulf, where Iran seized a British-flagged oil tanker in the Persian Gulf on Friday.
Theresa May will chair a meeting of the Government's emergency committee Cobra on Monday.
An intelligence ministry statement read on Iranian state TV said: "The identified spies were employed in sensitive and vital private sector centres in the economic, nuclear, infrastructural, military and cyber areas... where they collected classified information."
In June, Tehran claimed it had exposed a large cyber espionage network, alleging it was run by the CIA.
It claimed several US spies had been arrested in different countries as a result of the action.
It was unclear whether that case was linked to Monday's announcement of 17 arrests.
Hostilities between Iran and the West, mainly the US, have increased after Washington imposed new, tighter sanctions in May.
The sanctions were introduced after President Donald Trump pulled out of a nuclear deal signed by his predecessor Barack Obama.
The deal had provided Iran access to world trade in return for curbs on its nuclear programme.
In recent weeks, the US has accused Iran of seizing several oil tankers in the region, and the Americans and Iranians have shot down each other's drones.
Britain became involved when the Royal Marines seized an Iranian tanker off the coast of Gibraltar earlier this month as the UK claimed the vessel had violated sanctions on Syria.
Iran then made repeated threats of retaliation.
Members of its Revolutionary Guard boarded and seized the Swedish-owned, UK-flagged Stena Impero on Friday.
The cargo ship and its 23 crew members were taken to the Port of Bandar Abbas.
Iran claims the Stena Impero violated international maritime law. Britain said the ship was in Oman’s territorial waters when its was seized, and it was following international law.
UK ships have been warned to avoid the Strait of Hormuz, a key oil shipping route.
At Monday's Cobra meeting, Mrs May is expected to receive updates from ministers and officials on the seized tanker and will discuss the maintenance of the security of shipping in the region.
The Prime Minister was not present at ministerial Cobra meetings over the weekend - which she spent in her Maidenhead constituency - but was kept informed of developments.
Foreign Secretary Jeremy Hunt will update the Commons on the situation on Monday afternoon, amid reports that ministers are considering freezing Iranian regime assets.
He is expected to tell MPs what further measures the Government will take.
Hours before the meeting, defence minister Tobias Ellwood, speaking on ITV's Good Morning Britain, highlighted how the threat from Iran towards commercial shipping is an issue which does not just affect the UK.
"There is much criticism about the navy, about HMS Montrose not getting to there on time - I should actually point out that the USA have five or six warships in the region, including an aircraft carrier, and in the middle of June two of their tankers were attacked, one of them set ablaze," he said.
"So, this is something that affects us all, it requires an international co-operation, but also most importantly recognition that there's a bigger geostrategic challenge facing (us) here - that is the reason why Iran is doing those things."
With more "diverse and complex threats", Mr Ellwood, who continued his calls for more funding and investment for the Armed Forces, said the Iranian issue is an example of one of them.
Asked if Boris Johnson is the right man to de-escalate tensions with Iran if he becomes prime minister, Mr Ellwood said: "We have to back the winner, that is absolutely right."
Pressed on whether he will back Mr Johnson and stay in post, Mr Ellwood said: "It is every MP's duty to get behind this prime minister."
Former prime minister Tony Blair said the future of Iran's nuclear deal could be a way to exert diplomatic pressure on Tehran.
"We have one substantial card in our hands, which is that the Iranians have been trying to get the British and the Europeans to keep to the Iran nuclear deal," he told BBC Radio 4's Today programme.
"Therefore actually their attempt to interfere with a British-flagged ship is obviously wrong, not just in principle, but it's obviously a political gamble for them."
He added that the UK would have to make special arrangements to escort shipping through the Strait of Hormuz.
