In the latest example of ongoing hostilities despite repeated ceasefires and agreements, Israel launched a drone strike in Lebanon's south.
More than a million Lebanese have been driven from their homes by a conflict that has run in parallel with the wider Iran war.
Hezbollah and Iran say the United States pledged to end hostilities in Lebanon as part of its memorandum of understanding signed two weeks ago to end the wider war.
The framework agreed on Friday provides for a phased Israeli withdrawal from some parts of southern Lebanon, alongside the deployment of the Lebanese army.
But Israeli forces would be permitted to remain in an expanded security zone for the time being, pending further implementation.
In a statement, Qassem called it "null and void" and accused the Lebanese government of making unilateral concessions and undermining Lebanon's sovereignty.
He criticised provisions linking Israel's withdrawal to Hezbollah's disarmament, saying they effectively legitimised Israel's military presence and crossed "all red lines".
The group would continue its armed resistance, he added: "We did not leave the battlefield in the most difficult circumstances, and we will not leave it."
Lebanon's state news agency said an Israeli drone struck Nabatieh al-Fawqa on Saturday.
The area is outside the security zone shown on a map published by Israel of the territory its troops will continue to control.
The Israeli military told Reuters it had carried out the strike, using a drone because it had no troops in the immediate area.
It said it targeted an individual who posed a threat to its forces, without giving further details or evidence.
Qassem said the Iran-US memorandum of understanding reached earlier this month, which guarantees Lebanon's territorial integrity, should serve as the basis for ending the conflict rather than Friday's Washington DC agreement.
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