Shelling in the town of Herak, south of Damascus, killed seven people, six of them children, the Syrian Observatory for Human Rights said, taking the nationwide death toll for Tuesday to 33.

"At least seven civilians, including six children, were killed by regime forces shelling of the besieged town of Herak," the Britain-based watchdog said.

The Syrian government has moved chemical weapons to airports on its borders, the rebel Free Syrian Army said Tuesday, a day after the regime warned it could use them if attacked by an outside force.

"We in the joint command of the Free Syrian Army inside the country know very well the locations and positions of these weapons," a statement from the FSA said.

U.S. President Barack Obama warned Syrian leader Bashar Assad on Monday not to make the "tragic mistake" of turning to his stockpile of chemical weapons.

Assad's beleaguered regime had earlier threatened to unleash the weapons if Syria faced international military intervention, although it vowed not to turn them against its own civilians.

Syrian rebels "liberated" several districts of the northern city of Aleppo on Monday, a Free Syrian Army spokesman in the country's commercial hub said.

"This is the first step towards the liberation of the city," Colonel Abdel Jabbar al-Oqaidi told Agence France Presse via Skype, adding that fierce clashes continued on the edges of the Salah el-Din, Shaar, Hanano City, Tareq al-Bab and Sheikh Najjar neighborhoods.

The energy-rich Gulf Cooperation Council on Monday expressed support for Yemen's security days after Sanaa said it uncovered what it called an Iranian spy cell.

GCC secretary general Abdelatif Zayani praised Sanaa for dismantling the alleged spy ring and said the six-nation council "fully backs Yemen in all the measures it undertakes to reinforce stability and security," a statement said.

The United States warned Syria Monday not to even consider using chemical weapons after Damascus raised the possibility of employing its stockpile of unconventional arms in the case of an outside attack, as the EU branded the Syrian remarks as "monstrous" and "unacceptable."

"They should not think one iota about using chemical weapons," Pentagon press secretary George Little told reporters.

Russian President Vladimir Putin warned on Monday of a protracted civil war in Syria should President Bashar Assad be "unconstitutionally" removed from power by rebel fighters.

"We are afraid that if the country's current leadership is removed from power unconstitutionally, then the opposition and today's leadership may simply change places," Russian news agencies quoted Putin as saying.

Syrian regime forces on Monday reclaimed most of Damascus after a week of heavy fighting with rebels, who are using guerrilla tactics to keep a foothold in the capital, activists and regime sources said.

"Unfortunately, regime forces have taken control of almost all the capital. There are still skirmishes in the southern district of Qadam and Daraya," a suburb to the south, an activist who named himself as Omar told Agence France Presse via Skype.

Iraq on Monday rejected an Arab League call for embattled Syrian President Bashar Assad to swiftly give up power, describing it as interfering in Damascus's sovereignty.

"This call is not appropriate at this time because it is interfering in the sovereignty of another country," Deputy Foreign Minister Labid Abbawi told Agence France Presse. "There are other means to secure a peaceful transition of authority."

Turkey recalled its envoy in Aleppo on Monday as rebel fighters clashed with loyalist troops in what is Syria's second-largest city, a Turkish diplomat said.

Turkey's consul for the northern city of Aleppo, Adnan Kececi, had returned to Turkey to discuss the situation in Syria, the source said.