Israel "delivered crushing blows to all our enemies", Prime Minister Benjamin Netanyahu said, explicitly mentioning the killing of Hezbollah leader Fouad Shukur in south Beirut.

Netanyahu's televised statement on Wednesday lasted for approximately five minutes and did not make any reference to the killing of Hamas chief Ismail Haniyeh in Tehran.

An Israeli strike that killed a Hezbollah commander in Beirut and the killing of the political leader of Hamas in Tehran "don't help" regional tensions but there are no signs of an imminent wider conflict, the White House said.

"These reports over the last 24, 48 hours certainly don't help with the temperature going down," National Security Council spokesman John Kirby told reporters Wednesday when asked about the attacks.

U.N. Secretary-General Antonio Guterres has denounced strikes on Beirut and Tehran as a "dangerous escalation," after Israel targeted a top Hezbollah commander in Lebanon and Hamas said its political chief was killed in Iran.

"The scretary-general believes that the attacks we have seen in South Beirut and Tehran represent a dangerous escalation at a moment in which all efforts should instead be leading to a ceasefire in Gaza" and "the release of all Israeli hostages," spokesman Stephane Dujarric said in a statement.

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.