Abdelbasset Seida, head of the Syrian National Council opposition group, said, "A regime that massacres children and rapes women could use these types of weapons.
"The technical infrastructure may not be suitable, but as I said, such a step could be expected from this murderous regime. The international community must prevent this," he told reporters after meeting Turkey's foreign minister in Ankara.
Arab League ministers meeting in Doha urged the opposition and the rebel Free Syrian Army to form a transitional government, Qatari Prime Minister Sheikh Hamad bin Jassim al-Thani told a news conference.
Makdissi rejected the call for Assad to step down as a "flagrant intervention" in Syria's internal affairs. "We regret that the Arab League stooped to this immoral level," he said.
U.S. officials said on Monday the Obama administration was shifting its focus from deadlocked U.N. diplomacy over Syria and preparing to provide additional communications equipment and training to help the Syrian opposition improve its command-and-control capabilities for coordinating its fighters.
Officials insist that Washington has no plans for now to send lethal weapons to Syria's rebels, a step the White House has publicly ruled out.
On Monday, the army shelled rebel forces in the northern city of Aleppo and stormed the southern Damascus neighbourhood of Nahr Aisha, breaking into shops and houses and burning some of them, activists said.