Two back-to-back strikes in Beirut and Tehran, both attributed to Israel and targeting high-ranking figures in Hamas and Hezbollah, have left Hezbollah and Iran in a quandary.

Analysts agree that both strikes hit too close to home to pass without a response, and were serious security breaches for Iran and Hezbollah. Calibrating that response to restore deterrence without sparking an even more damaging escalation may be the most delicate balancing act in nearly a year of teetering on the brink of a regional war.

Iran held funeral processions on Thursday with calls for revenge after the killing in Tehran of Hamas political chief Ismail Haniyeh in a strike blamed on Israel.

The Islamic republic's supreme leader Ayatollah Ali Khamenei led prayers for Haniyeh ahead of his burial in Qatar, having earlier threatened a "harsh punishment" for his killing.

U.S. Secretary of State Antony Blinken on Thursday urged "all parties" in the Middle East to stop "escalatory actions" and achieve a ceasefire in Gaza, after Hamas's political leader was killed in a strike that Iran blamed on Israel.

Achieving peace "starts with a ceasefire, and to get there, it also first requires all parties to talk (and) to stop taking any escalatory actions", Blinken told reporters in Mongolia.

The targeting of two senior militant leaders in two Middle Eastern capitals within hours of each other — with each strike blamed on Israel — risks rocking the region at a critical moment.

The strikes come as international mediators are working to bring Israel and Hamas to agree to a cease-fire that would wind down the devastating war in Gaza and free hostages. Intense diplomatic efforts are also underway to ease tensions between Israel and Hezbollah after months of cross-border fighting.

The killing in Tehran of Hamas political leader Ismail Haniyeh, which the Palestinian militant group has blamed on Israel, has exposed the depth of Israeli penetration inside the Islamic republic, analysts say.

Doha-based Haniyeh, who the day earlier had attended the inauguration of new Iranian President Masoud Pezeshkian, was killed in the early hours of Wednesday morning at a residence in northern Tehran, according to Iran's Revolutionary Guards.

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: