An image grab taken from footage broadcast by Syrian state TV on July 22 purportedly shows Syrian security forces combing through the Basatin al-Razi dictrict of Damascus.
Syria admitted on Monday it possesses chemical weapons and warned it would use them if attacked though not against its own civilians, as regime troops battled rebels in Damascus and Aleppo.
The warning by foreign ministry spokesman Jihad Makdissi comes amid growing international concern that Damascus is preparing to deploy its chemical arsenal in the repression of a 16-month uprising against President Bashar al-Assad.
"Syria will not use any chemical or other unconventional weapons against its civilians, and will only use them in case of external aggression," Makdissi told a news conference.
"Any stocks of chemical weapons that may exist, will never, ever be used against the Syrian people," he said, adding that in the event of foreign attack, "the generals will be deciding when and how we use them."
Makdissi stressed later in an email that Syria would "never use chemical and biological weapons during the crisis... and that such weapons, if they exist, it is natural for them to be stored and secured."
Kassem Saadeddine, spokesman for the joint command of the rebel Free Syrian Army (FSA), said Makdissi's remarks gave cause for concern.
"The regime admits having chemical weapons, and as it has not signed any treaties. That proves that it will not hesitate to use them," Saadeddine told AFP.
Makdissi's comments come a day after the United States said it would "hold accountable" any Syrian official involved in the release or use of the country's chemical weapons.
The ministry spokesman also said Syria firmly rejected a demand by the Arab League that Assad step down.
"We are sorry that the Arab League has descended to this level concerning a member state of this institution," he said.
"This decision only concerns the Syrian people, who are the sole masters of the fate of their governments."
A meeting late Sunday in Doha of Arab League foreign ministers issued a statement calling on Assad to "renounce power," promising he and his family would be offered "a safe exit."
"There is agreement on the need for the rapid resignation of President Bashar al-Assad," Qatar's Prime Minister Sheikh Hamad bin Khalifa Al-Thani said after the meeting.
Makdissi also vowed Syrian forces would soon regain control of several border posts that rebel forces seized along the frontier with Iraq and Turkey.
The rebels "will not hold onto them and they will be gone in a few days," he said.
On the ground, Syrian regime forces raided several districts of Damascus on Monday, including Mazzeh, where where a plume of black smoke could be seen after night-time clashes in the capital, an AFP reporter said.
Activists reported clashes during the night in Syria's second city of Aleppo, and a rights watchdog reported 18 people killed Monday across the strife-torn country, including nine civilians, six soldiers and three rebels.
"Regime troops raided the edges of the Razi area of Mazzeh, Nahr Ayshe (southern Damascus) and Lawane in Kfar Sousa (in the southwest)," said the Syrian Observatory for Human Rights.
The watchdog added that troops proceeded "to carry out abuses against shopowners and inhabitants of the houses they raided."
State news agency SANA said the army had "restored security" in the Razi outskirts, "surrounding and killing several terrorists."
Elsewhere, rebels and troops clashes violently in Syria's commercial hub Aleppo, where the rebel FSA says a war of "liberation" is underway.
Clashes engulfed the eastern Sakhur and Hanano City districts, leading residents to flee the areas, the Observatory said.
Largely excluded from the violence and protests of the country's 16-month uprising until recently, Aleppo has emerged as a new front in the battle between rebel fighters and the regime.
Regime troops also used helicopters to pound the central city of Homs -- symbol of the uprising -- and nearby rebel-held Rastan on Monday, activists said.
The Observatory said on Monday that the death toll in fighting across Syria on Sunday stood at 123, including 67 civilians, 22 rebels and 34 soldiers.
The watchdog group said that more than 19,000 people have been killed since the uprising against Assad's regime began in March 2011.
The Britain-based Observatory meanwhile claimed at least 23 people were "summarily executed" by regime forces in Damascus.
"Sixteen people, most of them younger than 30, were summarily executed by shooting on Sunday in Mazzeh," Observatory chief Rami Abdel Rahman said. Seven others were executed in a similar fashion in Barzeh.
It was unclear whether the executions were of civilians or rebels fighters.
Fighting has intensified since a Wednesday bombing that killed national security chief General Hisham Ikhtiyar, Defence Minister General Daoud Rajha, Assad's brother-in-law Assef Shawkat and General Hassan Turkmani, head of the regime's crisis cell on the uprising.
The EU meanwhile beefed up sanctions against Assad's regime on Monday and agreed to tighten an arms embargo by inspecting vessels and planes suspected of carrying arms, diplomats in Brussels said.
EU foreign ministers began talks in Brussels with an agreement to freeze the assets of 26 Syrians and three firms close to the Assad regime in the 17th round of sanctions since protests erupted last year, diplomats said.