Israel's foreign minister, Avigdor Lieberman, said the transfer of Syrian chemical weapons to Hezbollah in Lebanon would be a cause for war.
Lieberman made the warning at a European Union meeting in Brussels, where officials are talking about tighter sanctions against Syria.
"In the moment we see that the Syrians transfer chemical and biological weapons to Hezbollah, this is a red line for us," Lieberman said.
If the transfer is made, "We will act decisively, and without hesitation or restraint," he continued. "It will be a completely different ball game and we hope for the understanding of the international community."
A Syrian foreign ministry spokesman admitted Monday that Syria has chemical weapons but said they would not be used against Syrian citizens.
On Tuesday, opposition leaders claimed that President Bashar Assad's forces had moved chemical weapons to airports on Syria's borders.