Both Hamas and Hezbollah issued condemnations on Tuesday of Israel's assassination of senior Iranian Revolutionary Guards Corps (IRGC) adviser Sayyed Razi Mousavi in Syria on Monday.
Hamas said in a statement that "the assassination of Brigadier General Radhi Mousavi in Syrian territory is a crime, a cowardly attack [which] threatens security and stability in the region".
Hezbollah similarly condemned the assassination, calling it a "flagrant and brazen attack and a transgression of borders", and praised Mousavi's support for the Lebanese armed group.
Hamas and Hezbollah are allied with Iran and comprise elements of the so-called "Axis of Resistance" along with Iran and Syria, as well as militias in Iraq and Yemen.
According to Iranian state media, Mousavi was one of the oldest advisers of the IRGC in Syria, while Reuters cited security sources saying he was responsible for the military alliance between Syria and Iran.
Iran helped prop up President Bashar al-Assad during Syria's twelve-year-long civil war, providing much of the ground troops that allowed Assad to retake the country from opposition fighters.
Mousavi was killed by three missiles on Monday after departing from the Iranian embassy in Damascus to his residence in the Sayyidah Zayneb suburb.
Iran's ambassador to Iran, Hossein Akbari, said that Mousavi was the second adviser at the Iranian embassy in Iran and a diplomatic passport holder, and said the assassination was an encroachment on Syrian sovereignty.
The IRGC threatened that Israel would "definitely pay the price for this crime" and promised a retaliation at a later time.
Analysts speculated that one of the theatres in which Iran would retaliate against Israel would be northern Israel, where Hezbollah and Israel have been engaged in over 10 weeks of border clashes.
Israeli officials said that they were preparing for a potential response from Syria or Lebanon over the killing of Mousavi.
On Tuesday, the group launched of series of attacks towards northern Israel, including one which injured at least nine Israeli soldiers.
Hezbollah's statement did not mention any threats towards Israel, however, the leader of Hezbollah Hassan Nasrallah is scheduled to speak on Wednesday, when he is expected to address Israel's assassination of the IRGC commander.
Syrian officials and state media were conspicuously silent about the assassination and did not mention the Israeli attack - contrary to its usual policy of announcing Israeli strikes in its territory.