Over 50 people were killed in fierce fighting between Syrian rebels and Hezbollah operatives near the border town of Qusair, including a senior Hezbollah field commander who spent time in Israeli jail, rebel sources in Syria reported Sunday night.

Fadi al-Jazzar is believed to have served 14 years in jail in Israel for a border attack in 1991, before being released in a prisoner exchange.

Earlier in the day, Syrian government troops, alongside Hezbollah fighters, launched a major assault on rebel forces around the strategic town near the Lebanese border.

The battle was described as the fiercest fighting involving Hezbollah in the two-year civil war. Over 50 shells were hitting the city a minute during bombardments by regime forces, an opposition activist told Reuters.

Jazzar, a Beirut native, was arrested by the Israeli military after launching the 1991 attack on the Israel-Lebanon border and sentenced later that year to life in prison for membership in an illegal organization, military training, and firing at a person, according to the Israeli Ministry of Justice website.

In early 2004, Jazzar was released by Israel, along with 434 other prisoners, including Mustafa Dirani and Abdel Karim Obeid, in exchange for Elhanan Tannenbaum, an Israeli businessman who was kidnapped by Hezbollah and held hostage for three years, and the bodies of IDF soldiers Benny Avraham, Adi Avitan, and Omar Sawaid.

According to Syrian opposition sources, the Syrian army soldiers, tanks and planes, supported by hundreds of Hezbollah fighters, pounded the rebel-held city Sunday, inflicting significant civilian casualties.

The Local Coordination Committees of Syria reported that 77 were killed across the country on Sunday, including 41 in Homs province, “most of them in Qusair.”

Syrian President Bashar Assad’s forces have besieged Qusair since November 2011. In recent weeks, a government offensive along the Lebanese border has attempted to regain control of the strategic area.

The city is important to both the rebels and the government. Qusair lies 35 kilometers southwest of Syria’s third-largest city, Homs, known as the “Capital of the Revolution,” and allows the rebels to maintain contact with their forces there.

For Assad, regaining control of Qusair is crucial to maintaining a corridor between Damascus and the Alawite mountains north of Lebanon. In addition, it lies on the frontier with Lebanon, home to Hezbollah, the pro-Assad Shiite terrorist organization which, rebels allege, has assumed an increasingly active role in the fighting.

“The Qusair area is highly important…” Elias Hanna, of the American University of Beirut, told AP. “It is the link between Damascus, Homs and the coastal area, so he can ill afford to lose it.”

A government official in Homs said the army massed its forces on three fronts around Qusair, while leaving a corridor for “safe passage for fleeing civilians and the armed terrorists who want to surrender.”

The official said government forces advanced into the town, taking over the municipality building and other vital government institutions.

But Hadi Abdullah, an activist in Qusair reached on Skype, denied the regime’s claim of advances. He said the municipality was destroyed in fighting six months ago, and that there was no government building left to take over.

The discrepancy in the accounts could not be immediately verified.

Syrian Army soldiers looking to defect have regularly headed to Qusair to link up with rebel forces. Pro-Assad fighters managed to capture parts of the city in the past, but were repulsed by Free Syrian Army counterattacks.

Some observers believe that controlling the region around Qusair is especially important for Assad’s long-term plans. Abdulrahman al-Rashid, writing in the London-based a-Sharq al-Awsat, suggested Assad might be rooting out opposition forces in western Syria in order to create a de-facto republic consisting of “Alawite, Christian, Druze and Shiite minorities in addition to Sunni categories affiliated with the regime.”

The offensive was taking place as Washington and Moscow attempt to bring representatives of the Syrian regime and opposition to the negotiating table at an international conference. On Friday, UN Secretary General Ban Ki-moon gave his support to the proposed conference in a meeting with Russian President Vladimir Putin.

The joint effort, the countries’ first concerted attempt to bring the fighting to an end in a year, aims to create a transitional government and institute a ceasefire. Russia supports the Assad government, while the United States backs the opposition.

In Israel, Prime Minister Benjamin Netanyahu vowed Sunday to continue to act to prevent advanced weaponry from being transferred to Lebanese terror group Hezbollah via Syria.

“The Israeli government has acted responsibly and prudently to ensure the security of Israeli citizens and to prevent advanced weapons from reaching Hezbollah and [other] terrorist organizations… and we will do so in the future,” Netanyahu said during the weekly cabinet meeting in Jerusalem.

Netanyahu’s remarks came after several days of increased tension between Israel and Syria. On Saturday, Assad accused Israel of backing and providing support to rebel factions, contradicting top defense official Amos Gilad, who on Friday stated that Israel was not seeking to topple Assad.

On Sunday, The Times of London reported that Syria had trained long-range missiles on Tel Aviv, to be used if Israel violates Syrian territory. Israeli jets reportedly struck sites near Damascus twice earlier in May, aiming to stop the transfer of advanced Fateh-110 missiles to Hezbollah.

Associated Press and Times of Israel staff contributed to this report.