Syria admitted 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 Syria's second-biggest city of Aleppo, however, as rights activists reported violence across the country killed at least 52 people, including 24 civilians.

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

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."

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."

The United States warned Syria over the use of chemical arms. "They should not think one iota about using chemical weapons," Pentagon press secretary George Little told reporters.

The White House has said Washington would "hold accountable" any Syrian official involved in the release or use of the country's chemical weapons.

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."

Ban also announced that UN peacekeeping chief Herve Ladsous as well as chief UN military adviser General Babacar Gaye were traveling to Syria on Monday.

Gaye is due to take over the command of the UN Supervision Mission in Syria (UNSMIS) which was extended with a "final" 30-day mandate last week by the Security Council.

On the political front, Makdissi 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."

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

Putin warned of a protracted civil war in Syria 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.

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

Prime Minister Nuri al-Maliki, meanwhile, instructed Iraqi 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 says a war of "liberation" is also underway.

Clashes engulfed the eastern Sakhur and Hanano City districts, leading residents to flee, according to the Syrian Observatory for Human Rights.

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

The Britain-based Observatory says more than 19,000 people have been killed since the uprising began in March 2011.

The EU 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 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 anti-regime protests erupted last year, they said.

EU officials 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.