Iran imposed sanctions on the US ambassador to Iraq, citing "terrorist" acts against its interests, in response to officials in Washington blacklisting Tehran's Baghdad envoy.
Matthew Tueller and two other US diplomats in Iraq "have effectively engaged in organising, financing, leading and committing terrorist acts against the interests of the government or citizens" of Iran, the foreign ministry said in a statement released on Twitter.
It also accused them of being involved in the killing of top Iranian general Qasem Soleimani by a US drone strike near Baghdad's airport in January, "support for extremist and terrorist groups", and implementing US sanctions against the Islamic republic.
The two other diplomats are Mr Tueller's deputy Steve Fagin and Rob Waller, head of US consulate in Arbil, the capital of Iraq's northern Kurdistan region.
"Anti-Iran moves won't go unanswered," Iran's foreign ministry spokesman Saeed Khatibzadeh wrote on Twitter with the statement attached.
Yesterday, the United States imposed new sanctions on five Iranian entities over "brazen attempts" to interfere with the November 3 US election, with Tehran strongly denying the accusations.
The US also separately blacklisted Tehran's envoy to Baghdad Iraj Masjedi, describing him as a "close adviser" to Soleimani.
In response, Mr Masjedi told state media he was "pleased to hear the news" that the "terrorist and criminal regime of the United States has put me once more on the list of its unjust sanctions alongside 80 million Iranians."
Tensions between the nations have increased since US President Donald Trump withdrew from a landmark nuclear agreement with Tehran and world powers and reimposed punishing sanctions.
They flared in January when a US drone strike killed Mr Soleimani, who headed the Quds Force of Iran's Revolutionary Guards.
US suspends visa services in Turkey
Elsewhere, the US Embassy in Ankara said it was temporarily suspending all American citizen and visa services at missions in Turkey over credible reports of potential terrorist attacks and kidnappings against US citizens in Istanbul.
"The US Mission in Turkey has received credible reports of potential terrorist attacks and kidnappings against US citizens and foreign nationals in Istanbul, including against the US Consulate General, as well as potentially other locations in Turkey," the embassy said.
A US Embassy spokesperson said the statement was issued after an ongoing assessment of security conditions in Turkey.
On New Year's Day 2017, a lone gunman killed 39 people in an attack at an Istanbul nightclub which was later claimed by Islamic State.
It was one of a series of militant attacks that killed dozens of people in Istanbul and elsewhere in Turkey.
Since the attack, Turkish authorities have cracked down on suspected Islamic State members and Kurdish militants, carrying out many security operations across the country and detaining hundreds of people.