The newly appointed U.N. envoy to Syria, Lakhdar Brahimi, will visit New York to meet with U.N. Secretary-General Ban Ki-moon and senior aides this week, a U.N. spokesman said.

U.S. President Barack Obama, meanwhile, has delivered a stern warning to al-Assad: Use chemical weapons, and risk provoking a military response.

"We have communicated in no uncertain terms with every player in the region that that's a red line for us and that there would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons," he said Monday.

Syria is a juggernaut when it comes to chemical weapons, analysts say.

The regime "probably has the largest and most advanced chemical warfare program in the Arab world," Michael Eisenstadt of the Washington Institute for Near East Policy said last month.

It includes thousands of rocket artillery rounds filled with mustard-type blister agents that can damage skin and lungs, and bombs filled with sarin that attacks the nervous system.

Syria has denied owning chemical or biological weapons. But Foreign Ministry spokesman Jihad Makdissi appeared to reverse that stance last month, issuing a veiled threat:

"No chemical or biological weapons will ever be used ... unless Syria is exposed to external aggression."

Al-Assad has long described the uprising against his rule as a terrorist revolt and a "foreign conspiracy."

The region: Fierce clashes erupt in neighboring Tripoli, Lebanon

Historic tensions between Sunni Muslims in Tripoli's Bab-al-Tibbaneh neighborhood and Alawite Muslims in the Jabal Mohsen neighborhood boiled over into clashes Tuesday.

Children were playing with BB guns, leaving one wounded, said Ali Fedda, a member of the Arab Democratic Party in the neighborhood. Residents then began shooting, and the army intervened in response, Fedda said.

In recent months, the Syrian crisis has aggravated decades-old quarrels between residents of the rival neighborhoods. Dozens of gunmen exchanged gunfire as the Lebanese troops responded to the violence Tuesday afternoon.

Two people were killed and 36 injured, Lebanese state-run media reported Tuesday.

The strife between Alawites and Sunnis in Tripoli mirrors the conflict in Syria, where al-Assad's regime is dominated by minority Alawites and the opposition is largely made up of Sunnis.