The pre-dawn killing of a top Hamas leader in Tehran, Iran, on Wednesday left the West Asia on edge, bringing vows of revenge from Iran’s leaders and threatening to derail fragile negotiations for a ceasefire in the Gaza Strip.

Ismail Haniyeh, 62, a top negotiator in the ceasefire talks who had led Hamas’ political office in Qatar, was killed after he and other leaders of Iranian-backed militant groups had attended the inauguration of Iran’s new President.

Israeli leaders would not confirm or deny whether their country was behind the brazen breach of Iran’s defences. But Iranian leaders and Hamas officials immediately blamed Israel and vowed to avenge the death of Haniyeh, heightening fears of a broader regional war.

Iran’s new President, Masoud Pezeshkian, said on Wednesday, a day after he was sworn into office with Haniyeh seated in the front row: “We will make the occupying terrorist regime regret its action. Iran will defend its sovereignty, dignity, reputation and honour.”

Hours before the killing of Haniyeh in the Iranian capital, Israeli fighter jets had carried out a separate operation in the southern suburbs of Beirut, Lebanon’s capital, and killed Fouad Shukur, a senior member of Hezbollah, the Lebanese militia that, like Hamas, is backed by Iran.

Hezbollah has been fighting a low-level war with Israel since October and has backed Hamas, which led a deadly rampage on southern Israel that precipitated the war.

Hezbollah confirmed on Wednesday that Shukur had been killed in the Israeli strike.

Haniyeh was killed in an explosion in a Revolutionary Guard guesthouse in Tehran where he was staying.

The two attacks shifted the calculus in the West Asia after Israel and Hamas had appeared to be edging closer to a ceasefire in Gaza, where tens of thousands of people have been killed over nearly 10 months of war.

Negotiators had hoped that such a deal would also lead to a truce between Israel and Hezbollah, which began firing into northern Israel in support of Hamas. John Kirby, a White House national security spokesperson, said the Biden administration believed it was “too soon to know” what effect the assassination might have on negotiations over a ceasefire and the release of hostages.

Four Syrians were killed and at least five Lebanese were hurt in an Israeli strike in south Lebanon, the health ministry said on Thursday.