The militant Palestinian group Hamas has a history of swift and smooth replacement of fallen leaders killed in Israeli airstrikes.

Ismail Haniyeh's assassination in the Iranian capital early Wednesday comes at a time when Hamas is under extreme pressure since the war in Gaza started nearly 10 months ago following the group's attack on southern Israel.

U.S. Secretary of State Antony Blinken said Wednesday that the United States was "not aware of or involved in" the killing of Hamas's political leader Ismail Haniyeh in Iran.

"I can't tell you what this means. I can tell you that the imperative of getting a ceasefire, the importance that that has for everyone, remains," Blinken said, according to a transcript shared by his staff from an interview with Channel News Asia in Singapore.

A pair of strikes on militant leaders in Beirut and Tehran has escalated tensions in a region already on edge and adds to a long list of targeted killings attributed to Israel.

Hamas said Israel was behind the assassination of its supreme leader Ismail Haniyeh in Iran's capital, although there was no acknowledgement from Israel. And Israel claimed responsibility for a strike on Fouad Shukur, a top Hezbollah commander in Beirut, that the military said killed him.

Qatar's Prime Minister Sheikh Mohammed bin Abdulrahman Al-Thani on Wednesday cast doubt on the success of future mediation between Israel and Hamas following the killing of the Palestinian group's political chief Ismail Haniyeh in Tehran.

"Political assassinations & continued targeting of civilians in Gaza while talks continue leads us to ask, how can mediation succeed when one party assassinates the negotiator on other side? Peace needs serious partners," Sheikh Mohammed, who has led Qatar's mediation efforts, wrote on X.

Hamas political leader Ismail Haniyeh was killed on Wednesday in an Israeli strike in Iran, the group said, in an act that has stunned the international community.

Israel has not commented on the death but many countries have warned that it will have a major impact on efforts to bring a ceasefire in the Gaza war. Here are the key reactions:

Hezbollah condemned Israel's killing of Hamas leader Ismail Haniyeh on Wednesday, warning his death will only increase the resolve of Iran-backed "resistance" groups.

Hamas said Haniyeh was killed in an Israeli strike in the Iranian capital, where he was attending the swearing-in of the country's new president, and vowed the act "will not go unanswered".

Ismail Haniyeh, Hamas' supreme leader in exile who landed on Israel's hit list after the militant group staged its surprise Oct. 7 attacks, was killed in an airstrike in the Iranian capital early Wednesday. He was 61.

Hamas said Haniyeh was killed at his residence in Tehran in an Israeli airstrike after he attended the swearing-in ceremony of Iran's new president. Israel has not commented on the accusation.

Iran’s President Masoud Pezeshkian condemned the killing of Hamas leader Ismail Haniyeh in Tehran on Wednesday.

He said in a post on the X platform that his country will defend its territorial integrity and make those responsible regret their actions.

The United Nations human rights office issued a report Wednesday saying Palestinian detainees taken by Israeli authorities since the Oct. 7 attacks have faced waterboarding, sleep deprivation, electric shocks and other torture and mistreatment.

The report on detention says Israel’s prison service held more than 9,400 “security detainees” as of the end of June, and some have been held in secret without access to lawyers or respect for their legal rights.

Hamas leader Ismail Haniyeh was killed by an airstrike in the Iranian capital, Iran and the militant group said early Wednesday, blaming Israel for a shock assassination that risks escalating the regional conflict on multiple fronts.

There was no immediate comment from Israel, which has vowed to kill Haniyeh and other Hamas leaders over the group's Oct. 7 attack on southern Israel. The strike came just after Haniyeh had attended the inauguration of Iran's new president in Tehran, Iran said.