"We believe it is our duty to seek revenge," says Khamenei according to the Iranian state news agency Irna.
The Revolutionary Guard in Iran also promises revenge. In a statement, they say that the act will be answered "hard and painfully".
The terrorist-stamped Islamist movement blames Israel for the death and says it will not go unanswered. Israel, on the other hand, does not comment on the event.
Iran's new president, Masoud Pezeshkian, who has just taken office, writes on the platform X that his country will defend its territorial integrity and that those behind the attack "will regret their cowardly act".
US Secretary of State Antony Blinken, who is on a trip to Asia, tells the Singaporean TV channel CNA that the US "had no knowledge of or involvement" in the attack on Haniya.
I don't think I'll speculate on what impact the event may have, says Blinken, adding that they will continue to work on preventing an escalation of the conflict.
"Dangerous escalation"
Qatar, where Hamas' political leadership is based and which has mediated a ceasefire in the Gaza War, calls the attack a "heinous act" and a "dangerous escalation" of violence in the region.
"How can we succeed in mediating when one party murders the negotiator on the other side?", writes Qatar's prime minister on the platform X.
Hamas' close ally in Lebanon, Hezbollah, warns that Haniya's death will increase the determination of Iran-backed "resistance groups".
Several condemnations
Turkey, which as recently as Sunday threatened to attack Israel, condemns the "shameful murder" of the Hamas leader, writes the country's foreign ministry in a statement. The ministry also writes that Israel is trying to make the Gaza War spread regionally.
Russia also condemns the attack, calling it an "unacceptable political murder". Tensions will now rise further, according to Moscow.
Liu Jian, spokesperson for the Chinese Foreign Ministry, says that the Beijing regime is "very concerned about what has happened".
The Iran-backed Houthi movement, which controls large parts of Yemen, calls the attack a "terrorist crime".
