Iran's Revolutionary Guards said Wednesday that the chief of the Palestinian group Hamas, Ismail Haniyeh, was killed in Tehran along with one of his bodyguards.
"The residence of Ismail Haniyeh, head of the political office of Hamas Islamic Resistance, was hit in Tehran, and as a result of this incident, him and one of his bodyguards were martyred," said a statement by the Islamic Revolutionary Guard Corps's Sepah news website.
The cause of the "incident" was not immediately clear but the Guards said it was "being investigated."
Haniyeh arrived in Tehran on Tuesday to attend the inauguration of Iran's new President Masoud Pezeshkian in parliament.
He had met with Pezeshkian as well as Iran's supreme leader Ayatollah Ali Khamenei.
Haniyeh, who resided in Qatar, had long been a target for Israel, with Tel Aviv having killed three sons and four grandchildren of the Hamas leader in Gaza in April.
Tuesday's swearing-in ceremony came amid concerns of war between Israel and Lebanon's Hezbollah, who have exchanged fire since the beginning of Israel's latest war on Gaza in October.
Later on Tuesday, Israel struck Hezbollah's stronghold in southern Beirut, saying it had killed the commander responsible for the attack for the recent attack on the Israeli-occupied Golan Heights.
A source close to Hezbollah said Shukr was the target but that he "survived the Israeli strike". AFP was not immediately able to confirm that report.
Iran, which does not recognise Israel, has repeatedly warned Israel against attacking Lebanon.
"The Zionist regime (Israel) will make a great mistake with heavy consequences if it attacks Lebanon," Pezeshkian said during a Monday call with French President Emmanuel Macron, according to the Iranian president's website.
Regional tensions have soared since the start of the Israel's latest war on Gaza, drawing in Iran-backed militant groups in Syria, Lebanon, Iraq and Yemen.
Israel's war on Gaza has killed at least 39,400 people, according to the territory's health ministry.
Iran has made support for the Palestinian cause a centrepiece of its foreign policy since the 1979 Islamic revolution.