Hamas political chief Ismail Haniyeh was assassinated by a concealed explosive device smuggled into the guesthouse in the Iranian capital where he was staying, a report claimed on Thursday.

Haniyeh and a bodyguard were killed Wednesday in a predawn strike on their accommodation in Tehran, Iran's Revolutionary Guard Corps said, in an attack that has stoked fears of a wider regional conflict.

The bomb in the residence was hidden approximately two months ago, The New York Times, citing seven Middle Eastern officials, including two Iranians and an American official, said.

Haniyeh was in the Iranian capital for the inauguration of President Masoud Pezeshkian. The killing has sent shockwaves around the world and prompted major protests in the region.

Two members of the Islamic Revolutionary Guard Corps who were briefed on the incident said the explosion rocked the building, shattered windows and caused part of an exterior wall to collapse.

Iranian officials and Hamas said Wednesday that Israel was behind the killing, but the Times said that conclusion was also reached by several U.S. officials, who the newspaper said requested anonymity.

Israel has not yet claimed responsibility for the death of the Hamas leader.

The report, however, said Israeli intelligence officials briefed the U.S. and other Western governments on details of the operation in its immediate aftermath.

U.S. Secretary of State Antony Blinken earlier called on all parties in the Middle East to de-escalate tensions and urgently achieve a cease-fire in the Gaza Strip, where Israel has killed more than 39,000 Palestinian victims since Oct. 7.

Haniyeh has been Hamas's lead negotiator in efforts for a truce and hostage release deal for Gaza.