Muhammad Deif, leader of Hamas's military wing, published a letter Thursday morning sending his condolences to Hezbollah Secretary General Hassan Nasrallah.
The Al-Qassam Brigades chief commander has not been seen publicly since Operation Protective Edge last summer, in which he was reported to have been killed.
In the letter published on the Hezbollah-affiliated Al-Manar website, Deif sends comfort and blessings to Nasrallah and other top Hezbollah leaders following the deaths of six of their members by an Israeli air strike in Syria on Monday.
Jihad Mughniyeh, son of the late Imad Mughniyeh who was Hezbollah's former military commander, was among those killed, as was a Hezbollah commander. Six Iranian soldiers and commanders, including a general were also killed in the strike.
Deif stressed that the blood of these martyrs is a "resounding call and stinging blow to those who are demoralized and wish to surrender," likely a reference to the Palestinian Authority and their Chairman Mahmoud Abbas who are not engaged in a military struggle with Israel.
Additionally, Deif noted called for "all weapons to be directed toward Israel."
"There is an obligation on all opposition forces to challenge the Zionist enterprise and put up a strong unified hand against the Zionist enemy and its partners."
During Operation Protective Edge an Israeli air strike struck Deif's house killing his wife and child. Fox News reported Deif had been killed as well – a claim Hamas has consistently refuted.
In a recent video released for Hamas's 27th anniversary, an old recording of Deif from Operation Pillar of Defense in November 2012 can be heard. A short time later, the shadow of a man, similar in appearance to Deif, can be seen, despite blurry photography.
Deif has reportedly survived five Israeli air strikes. His location remains unknown.