"We consider the assassination of Brigadier General Seyyed Razi Mousavi in Syria to be a flagrant and shameless violation, which is off limits," Hezbollah said in a statement on Monday.
The senior IRGC commander was martyred in a missile attack by the Israeli regime in the Sayyeda Zeinab neighborhood of Damascus earlier in the day.
Hezbollah added that Mousavi "dedicated decades of his honorable life to support the Islamic resistance in Lebanon and tirelessly helped the resistance and its fighters."
The movement then described the martyred Iranian commander as "comrade in arms" of the former commander of the Islamic Revolution Guards Corps (IRGC)’s Quds Force Lieutenant General Qassem Soleimani, who was assassinated by the United States' terrorist forces in Iraq four years ago.
The movement concluded its statement by extending its condolences to Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei, the IRGC, Mousavi's family and the Iranian nation.
The Gaza Strip-based Palestinian Islamic Jihad also strongly condemned "the cowardly assassination" of the Iranian commander by the Israeli regime, calling Mousavi "one of the most prominent commanders" of the IRGC.
Iran's Foreign Minister Hossein Amir-Abdollahian also expressed his condolences to Mousavi’s family as well as the Iranian and Syrian nations on the occasion of his martyrdom.
In a post on X social media platform, Iran's top diplomat said, “This honorable martyr fought bravely against terrorists for many years alongside Martyr Hajj Qassem Soleiman to ensure security of Iran and the [West Asia] region.”
“Tel Aviv must await a hard countdown,” Iran's foreign minister added.
Mousavi was martyred while serving as part of Iran's military advisory mission in Syria.
The mission was the first to rush to Syria's assistance in 2014, when the Arab country found itself in the grip of the US-created and Israeli-supported Daesh Takfiri terrorist group.
Led by General Soleimani, the mission played an instrumental role in Damascus' successful counterterrorism push that ultimately defeated the terror outfit in late 2017.
Earlier on Monday, Iranian President Ebrahim Raeisi vowed that the Israeli regime would definitely pay the price for its criminal act of killing the senior military advisor.
Raeisi said the vicious act of assassinating the IRGC commander was yet another sign of the usurping Zionist regime's desperation.
Iran's Foreign Ministry spokesman Nasser Kan'ani also strongly condemned Israel's terrorist measure, saying the vicious and cowardly act was another indication of the terrorist nature of the regime.