Hezbollah leader Naim Qassem on Monday rejected Lebanon's planned direct negotiations with Israel, calling them a "grave sin" that will destabilize Lebanon.
Lebanon and Israel's US ambassadors held two meetings in Washington over the past weeks, the first of their kind in decades.
The first meeting led to a truce in the Israel-Hezbollah war, while Beirut has been preparing for direct negotiations with the aim of striking a peace deal with Israel. The two countries have officially been at war since 1948.
"We categorically reject direct negotiations with Israel, and those in power should know that their actions will not benefit Lebanon or themselves," Qassem said in a statement, aired by the group's channel Al-Manar.
He called on authorities to "back down from their grave sin that is putting Lebanon in a spiral of instability".
He added that the Lebanese government "cannot continue while it is neglecting Lebanon's rights, giving up land, and confronting its resistant people".
Lebanese authorities have repeatedly stated that the goal of the US-sponsored negotiations is to stop the war, secure Israel's withdrawal from southern Lebanon, and return displaced people to their homes after the fighting forced more than a million people to flee.
"These direct negotiations and their outcomes are as if they do not exist for us, and they do not concern us in the slightest," Qassem said.
"We will continue our defensive resistance for Lebanon and its people," he added.
"No matter how much the enemy threatens, we will not back down, we will not bow down, and we will not be defeated.
"We will not give up our weapons... and the Israeli enemy will not remain on a single inch of our occupied land."
Tehran-backed Hezbollah drew Lebanon into the Middle East war on March 2 by firing rockets at Israel to avenge the death of Iran's supreme leader Ali Khamenei in US-Israeli strikes.
Since the truce went into force on April 17, Israeli strikes have killed at least 36 people, according to an AFP tally of Lebanese health ministry figures.
Hezbollah has meanwhile claimed several attacks on Israeli troops in southern Lebanon, as well as missile and drone launches at northern Israel, saying it is responding to Israeli "violations".
According to details of the truce released by the US State Department, which said both Lebanon and Israel agreed to it, Israel reserves the right to continue targeting Hezbollah to prevent "planned, imminent, or ongoing attacks".
Hezbollah strongly rejects this clause, saying the text of the agreement was not presented to the cabinet, in which the group and its allies are represented.
"Has the government decided to work alongside the Israeli enemy against its own people?" Qassem said in his speech.
Israeli attacks on Lebanon killed more than 2,500 people since March 2, according to Lebanese authorities.