Syria admitted on Monday it has chemical weapons and warned of using them if attacked, though not against its own civilians, as regime troops reclaimed most of Damascus after a week of heavy clashes.

Fighting was still raging in the country's second city of Aleppo where rebels claimed to have captured several districts, however, and activists said at least 52 people, including 24 civilians, were killed in violence across Syria.

At a Damascus news conference, foreign ministry spokesman Jihad Makdissi warned Syria would use chemical weapons if attacked by outsiders, although he backtracked later to say that, if Damascus has them, they would be secured.

His remarks come amid growing international concern that Damascus is preparing to deploy its chemical arsenal in the repression of a 16-month uprising against the Assad regime.

"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 reporters.

"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."

US President Barack Obama responded swiftly, warning Syrian strongman Bashar al-Assad not to make the "tragic mistake" of deciding to unleash his stockpile of chemical weapons.

"Given the regime's stockpile of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching," Obama told an audience of US veterans in the western US state of Nevada.

"They will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons," he added.

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."

A leading Syrian opposition leader warned however that the Assad regime was indeed capable of using chemical weapons.

"A regime that massacres children, that rapes women, can just as well use chemical weapons," Abdel Basset Sayda told the Anatolia news agency.

UN chief Ban Ki-moon said the use of chemical arms would be "reprehensible" and stressed "all the countries have an obligation not to use any weapons of mass destruction, whether they are parties or not to any convention or agreement."

'Long civil war'

Meanwhile President Vladimir Putin of Russia, the Syrian regime's main international ally, warned of a protracted civil war should rebels be allowed to remove Assad from power.

"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," Interfax news agency quoted Putin as saying.

In that case "a civil war will stretch on for who knows how long," the Russian leader warned.

Syrian spokesman Makdissi said Damascus 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."

The Arab League had called late Sunday on Assad to "renounce power," promising he and his family would be offered "a safe exit."

Iraq also rejected the Arab League call for Assad to step aside, describing it as interference.

Iraqi Prime Minister Nuri al-Maliki, meanwhile, instructed the country's forces and the Red Crescent to allow in Syrian refugees and provide them with support, reversing an earlier decision.

Tens of thousands of Syrians have fled for neighbouring countries in recent days after violence mounted, particularly in Damascus.

On the ground, government forces reclaimed most of Damascus, after a week of heavy fighting with rebels, who remain in the city but are planning a guerrilla strategy, activists and regime sources said.

A security source in Damascus confirmed the city had been reclaimed by government forces.

Elsewhere, rebels and troops clashed violently in Syria's commercial hub Aleppo, where the rebel Free Syrian Army (FSA) says a war of "liberation" is also underway.

The regime also used helicopters to pound the central city of Homs -- symbol of the uprising -- and nearby rebel-held Rastan, activists said.

The Syrian Observatory for Human Rights said that 54 people died in Syrian fighting on Monday, adding to the 19,000 whom the British-based watchdog says have been killed since the uprising began in March 2011.

The European Union on Monday beefed up sanctions against Assad's regime and agreed to tighten an arms embargo by inspecting vessels and planes suspected of carrying arms, diplomats said.

EU foreign ministers, meeting in Brussels, agreed to freeze the assets of 26 Syrians and three firms close to the Assad regime in the 17th round of sanctions imposed since anti-regime protests erupted last year.

EU officials also said the bloc was ready to evacuate its nationals from Syria via Cyprus if necessary. "We are prepared for the worst," said Cypriot Home Affairs Minister Eleni Mavrou, whose country currently holds the EU helm.