Hassan Nasrallah spoke during an event commemorating the death of leading Hezbollah figure Hassan Hussein al-Hajj, known as Abu Muhammad.
According to Al-Manar television, which aired the speech, al-Hajj died on October 10 fighting in Sahl al-Ghab, a strategic plain in northwest Syria where Hezbollah has dispatched fighters in support of the embattled regime of Bashar al-Assad.
Nasrallah said Hezbollah members fought "for 30 years on the border with Palestine and have now gone to Sahl al-Ghab, where Abu Mohammad died, to the borders with (Syrian provinces) Hama, Idlib, Latakia, and Aleppo."
His group's presence in Syria "is larger than ever before -- qualitatively, quantitatively, and in equipment, because we are in a critical and definitive battle", Nasrallah said.
Hezbollah has sent thousands of fighters to support the Syrian army's ground offensives in the country's centre, north, and northwest, recently bolstered by Russian air strikes.
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But the group has been criticised for diverting resources from combatting its traditional enemy to the south, Israel, which Hezbollah most recently clashed with in a month-long war in 2006.
Nasrallah tried to temper that criticism by saying Israel and extremist factions in Syria, like the Islamic State group, had the same goals.
"The two projects -- the Zionist and the takfiri (extremist Sunni) -- want to achieve the same result: destroying our peoples and our societies," he said.
He said Hezbollah would continue to protect Lebanon from Israel, just as it was fighting extremist factions in Syria.
"If it were not for the perseverance on the ground in the face of Daesh (IS) and its brothers... then from Iraq, to Syria, to Lebanon, where would the region be today?" Nasrallah said.
He acknowledged that Hezbollah's fight in Syria "may be long," but said it would be necessary to "protect these countries and this region."