The predawn killing of a top Hamas leader in Tehran on Wednesday left the entire Middle East on edge, bringing vows of revenge from Iran’s leaders and threatening to derail fragile negotiations for a Gaza cease-fire.

The Hamas leader, Ismail Haniyeh, 62, a top negotiator in the cease-fire talks who had led the militant group’s 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 defenses. But Iranian leaders and Hamas officials immediately blamed Israel and vowed to avenge the death of Mr. Haniyeh, heightening fears of a broader regional war.

Iran’s supreme leader, Ayatollah Ali Khamenei, issued an order for Iran to strike Israel directly, according to three Iranian officials briefed on the order.

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