Hamas leader Ismail Haniyeh was assassinated in Tehran, Iran's paramilitary Revolutionary Guard said early Wednesday.

No one immediately claimed responsibility for the assassination but suspicion immediately fell on Israel, which has vowed to kill Haniyeh and other leaders of Hamas over the group's Oct. 7 attack on Israel that killed 1,200 people and saw some 250 others taken hostage.

Haniyeh was in Tehran to attend Iran's President Masoud Pezeshkian's swear-in ceremony on Tuesday. Iran gave no details on how Haniyeh was killed, and the Guard said the attack was under investigation.

Analysts on Iranian state television immediately began blaming Israel for the attack.

Israel itself did not immediately comment but it often doesn't when it comes to assassinations carried out by their Mossad intelligence agency.

There was no immediate reaction from the White House. The apparent assassination comes at a precarious time, as the Biden administration has tried to push Hamas and Israel to agree to at least a temporary cease-fire and hostage-release deal.

CIA Director Bill Burns was in Rome on Sunday to meet with senior Israel, Qatari and Egyptian officials in the latest round of talks. Separately, Brett McGurk, the White House Coordinator for the Middle East and North Africa, is in the region for talks with U.S. partners.

Israel is suspected of running a yearslong assassination campaign targeting Iranian nuclear scientists and others associated with its atomic program. In 2020, a top Iranian military nuclear scientist, Mohsen Fakhrizadeh, was killed by a remote-controlled machine gun while traveling in a car outside Tehran.

In Israel's war against Hamas since the October attack, more than 39,360 Palestinians have been killed and more than 90,900 wounded, according to the Gaza Health Ministry, whose count does not differentiate between civilians and combatants.