Hezbollah leader Sayyed Hassan Nasrallah denied on Monday that his group had received chemical weapons from Syria.
Last month, members of the Syrian National Coalition opposition group accused President Bashar Assad of transferring chemical weapons to the Lebanese Shi'ite group to avoid inspection after agreeing to put them under international control.
"This accusation is truly laughable," Nasrallah said in a televised speech. "We understand the dimensions and background of these accusations, and these have dangerous consequences for Lebanon.
"We decisively and conclusively deny these accusations which have absolutely no basis in truth."
Syria has agreed to give up its chemical weapons under a plan agreed by the United States and Russia after Western powers blamed Assad's government for a chemical attack that killed hundreds of people in a Damascus suburb last month.
Israel's commander on the frontier with Syria, Major-General Yair Golan, said this month that Hezbollah sought precision ground-to-ground rockets, anti-aircraft and anti-ship missiles from Syria in return for helping Assad, but "as far as we can tell" it did not want his chemical weapons.
Nasrallah called for a political solution in Syria and urged Sunni powers Turkey, Saudi Arabia and Gulf Arab countries that have backed the rebels to "review their positions".
"A gamble on a military resolution and on military success is a losing and destructive gamble," he said.
The intervention of Hezbollah fighters in Syria has raised fears among some Lebanese that the small Mediterranean country could become engulfed by its much larger neighbor's conflict, which has killed over 100,000 people.
Rocket and bomb attacks have hit Hezbollah strongholds in the Bekaa Valley and in the capital - the worst of which was a car bomb that killed 20 people in southern Beirut last month.
Nasrallah accused a radical Sunni Islamist group "working in the framework of the Syrian opposition" of carrying out the attack. He said the group had Lebanese and Syrian members but did not name it.
Earlier on Monday, Lebanese security forces replaced Hezbollah forces at checkpoints in the southern suburb which the Shi'ite group had set up after the car bomb.
Nasrallah said the group welcomed the deployment of the security forces and urged residents to cooperate with them.
Additional reporting by Laila Bassam and Erika Solomon; Editing by Angus MacSwan and Robin Pomeroy