In remarks broadcast on the Shiite group's Al-Manar television, Hassan Nasrallah said his party would "increase our presence in Aleppo... Because the real, strategic, greatest battle is in Aleppo and the surrounding area."
He spoke at a commemoration event for Mustafa Badreddine, a Hezbollah commander killed in an explosion on May 12 near Damascus international airport.
Hezbollah has intervened in Syria's complex war on behalf of the regime of President Bashar al-Assad and has dispatched thousands of fighters to Damascus, Aleppo, and Homs provinces, among others.
Regime fighters, backed by Hezbollah and government and Russian warplanes, are seeking to close off the last rebel route out of the city.
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"We are facing a new wave, or a new phase, of military operations in Syria that will be fought in the north, specifically in the area of Aleppo," Nasrallah said.
He announced that 26 Hezbollah fighters had been killed in the offensive this month, in a rare admission of casualties for the group.
"This is why it is our duty to be in Aleppo -- and we were, and we will stay in Aleppo.