The death of Ismail Haniyeh, the Hamas political leader, in Iran "will strengthen the deep and unbreakable bond between the Islamic Republic of Iran and the beloved Palestine and the resistance," an Iranian state spokesperson said.

"The pure blood of Martyr Haniyeh will never be wasted," Foreign Ministry spokesperson Nasser Kanaani said, according to Iranian state media.

Iranian authorities were investigating the assassination, he said.

Kanaani praised Haniyeh for spending his life in the "honorable struggle against the usurping Zionist regime" and for seeking the "the liberation of the oppressed Palestinian nation."

-ABC News' Somayeh Malekian and Joe Simonetti

Reaction from Hamas and its allies was swift, with Musa Abu Marzouq, a member of the Hamas political office, saying in a statement, "The assassination of Ismail Haniyeh, the head of the Hamas Political Bureau, is a cowardly act and will certainly not go unanswered."

Islamic Jihad also issued a statement, saying the death of Haniyeh will not deter them.

Mahmoud Abbas, president of the state of Palestine, said he strongly condemned the assassination of Haniyeh, calling the attack "a cowardly act."

-ABC News' Will Gretsky

Ismail Haniyeh, the political leader of Hamas, has been killed in Tehran along with his bodyguard, Iran’s Islamic Revolutionary Guard Corps has said in a statement.

There has been no claim of responsibility for the attack at this stage.

The Israel Defense Forces said it targeted and killed a senior Hezbollah commander in Beirut, Lebanon. The IDF said the commander is responsible for Saturday's strike that killed children playing soccer in Majdal Shams in the Israeli-controlled Golan Heights.

Israeli Defense Minister Yoav Gallant said on social media that Hezbollah "crossed the red line."

The Lebanese Red Cross said the strike hit a residential building, killing at least two and injuring 20.

The target of Israel's strike was Fouad Shukr, also known as Al-Hajj Mohsen, according to three security sources familiar with the operation.

The IDF claims Shukr was killed in the strike.

The United States was given advanced notice ahead of Israel’s strike, according to a U.S. official familiar with matter. The message was communicated via security channels and limited operational detail was shared, the official said.

Vice President Kamala Harris said Tuesday that "Israel has the right to defend itself against a terrorist organization, which is exactly what Hezbollah is."

"But all of that being said, we still must work on a diplomatic solution to end these attacks and we will continue to do that work," she added.

State Department deputy spokesperson Vedant Patel told reporters he didn’t have "any updates on any specific activity," but added, "We have been in continuous discussions with Israeli and Lebanese counterparts since the incident over the weekend, and the United States is going to continue to support efforts to reach a diplomatic solution along the blue line."

"Our support for Israel's security is ironclad, and it's unwavering, especially as it defends itself against Iran-backed threats, including threats from Hezbollah," Patel said.

-ABC News' Dana Savir and Shannon Kingston