Hamas leader Ismail Haniyeh assassinated in Tehran.

Hamas leader Ismail Haniyeh was assassinated in the Iranian capital of Tehran early Wednesday morning, the Palestinian terror group said, noting the killing could drastically change the outlook of Israel's war on Hamas.

Hamas deputy chief in Gaza Khalil Al-Hayya, citing witnesses who had been with Haniyeh in Iran, later said that the missile that killed Haniyeh struck him "directly," Reuters reported.

As a result of the missile strike, windows, doors, and walls in his room were destroyed, Al-Hayya told a press conference in Tehran.

Haniyeh, along with one of his guards, was killed at around 2:00 a.m. at his Tehran residence following the launch of projectiles from an adjacent building, according to Al-Arabiya and other Arab outlets.

Hamas claimed Israel was responsible for the targeted killing, saying in a statement that their political leader was killed in "a treacherous Zionist raid on his residence in Tehran."

Iran's Revolutionary Guards confirmed the death of Haniyeh hours after he attended a swearing-in ceremony for the country's new president and said it was investigating

As of Wednesday afternoon, Israeli officials have yet to take responsibility for the assassination. The IDF said it was assessing the situation but had not issued any new security guidelines for civilians.

Hamas' armed wing said in a statement Haniyeh's killing would "take the battle to new dimensions and have major repercussions." Vowing to retaliate, Iran declared three days of national mourning and said the US bears responsibility because of its support for Israel.

Musa Abu Marzouk, a senior member of Hamas's political bureau, warned that the elimination of the Haniyeh "will not pass in silence."

Iran's Supreme Leader Ali Khamenei said Israel had provided the grounds for "harsh punishment for itself" and it was Tehran's duty to avenge the Hamas leader's death as it had occurred in the Iranian capital. Iranian forces had already made strikes directly on Israel earlier in the Gaza war.

"With condolences to the heroic nation of Palestine and the Islamic nation and the combatants of the Resistance Front and the noble nation of Iran, this morning [Wednesday], the residence of Mr. Dr. Ismail Haniyeh, the head of the political office of the Islamic Resistance of Hamas, was hit in Tehran, and following this incident, he and one of his bodyguards were martyred," an IRGC statement read. "Early this morning, the residence of Ismail Haniyeh in Tehran was struck, resulting in his and one of his bodyguards' martyrdom. The cause is under investigation and will be announced soon."

Qatar, which has been brokering talks aimed at halting the fighting in Gaza along with Egypt, condemned Haniyeh's killing as a dangerous escalation of the conflict. Egypt said it showed a lack of political will on Israel's part to calm tensions. China, Russia, Turkey and Iraq also condemned it.

The assassination, which took place less than 24 hours after Israel claimed to have killed the Hezbollah commander it said was behind a deadly strike in the Golan Heights, appears to set back chances of any imminent ceasefire agreement in the 10-month-old war Gaza.

Iran's top security body met to decide strategy in reaction to the death of Haniyeh, a source with knowledge of the situation said.

Yoav Gallant: Israel is not trying to escalate the war
Defense Minister Yoav Gallant said Israel was not trying to escalate the war but was prepared for all scenarios. Visiting an air defense battery, he praised the forces who carried out the attack on the Hezbollah commander in Beirut.'

"Israel is not interested in an all-out war," Foreign Minister Israel Katz also stated in a message he sent to foreign ministers around the world on Wednesday.

In Israel, the mood was buoyant as Israelis welcomed what they saw as a major achievement in the war against Hamas, though residents in besieged Gaza feared Haniyeh's death would prolong the fighting that has devastated the enclave.

Three of Haniyeh's sons were killed in an airstrike in April. His sister was also eliminated last month. The Gaza-based residence of Haniyeh, which was used as terrorist infrastructure, was hit by an IDF airstrike in November. Haniyeh usually resides in Qatar, but his property was being used as a meeting place for other members of Hamas leadership.