The Islamic State group said Thursday that it carried out twin bombings which killed at least 84 people at a memorial ceremony in Iran for slain Revolutionary Guards general Qassem Soleimani.
The claim from IS came as Iran observed a day of national mourning for those killed in Wednesday's blasts.
In a statement on Telegram, IS said two of its members "activated their explosives vests" among the crowds who had come to honour Soleimani on the anniversary of his death in a targeted US drone strike in Baghdad four years ago.
Iranian investigators had already confirmed that the first blast at least was the work of a "suicide bomber" and believed the trigger for the second was "very probably another suicide bomber", the official IRNA news agency reported earlier, citing an "informed source".
Soleimani, who headed the Guards' foreign operations arm the Quds Force, was a staunch enemy of IS, the extremist group which has carried out previous attacks in Iran.
The death toll was revised down from around 100 the day after what Iranian authorities labelled a "terrorist attack" that also wounded hundreds near Soleimani's tomb in the southern city of Kerman.
Iran has suffered deadly attacks in the past from extremists and other militants as well as targeted killings of officials and nuclear scientists blamed on arch foe Israel.
On Thursday, Interior Minister Ahmad Vahidi spoke to ISNA news agency about bolstering security over its porous borders with Afghanistan and Pakistan.
He said authorities have identified "priority points to block along the border" with the two countries, which has long been a key access point for militant groups, drug smugglers and irregular migrants.
Iran's supreme leader Ayatollah Ali Khamenei on Wednesday blamed "evil and criminal enemies" of the Islamic republic, without naming them, and vowed a "harsh response".
Regional tensions have surged amid Israel's war on Gaza, which has killed over 22,000 people since October 7.
President Ebrahim Raisi's deputy chief of staff for political affairs, Mohammad Jamshidi, charged on social media platform X that "the responsibility for this crime lies with the US and Zionist (Israeli) regimes, and terrorism is just a tool".
The United States rejected any suggestion that it or its ally Israel were behind the bombings, while Israel declined to comment.
"The United States was not involved in any way, and any suggestion to the contrary is ridiculous," said State Department spokesman Matthew Miller.
"We have no reason to believe that Israel was involved in this explosion," he added, expressing sympathies to the victims of the "horrific" explosions and their families.