Damascus - US President Barack Obama has warned Syrian strongman Bashar Assad it would be a "tragic mistake" to unleash chemical weapons, after Damascus admitted it has a stockpile and will use them if attacked.
The warning, a first admission by the Syrian regime that it has weapons of mass destruction, came as regime troops reclaimed to control 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, and activists said at least 52 people, including 24 civilians, were killed in violence across Syria.
At a Damascus news conference on Monday, foreign ministry spokesperson Jihad Makdissi said Syria has chemical weapons and will use them if attacked by outsiders, though not against its own civilians.
Makdissi's remarks came 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.
'The world is watching'
"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".
President Obama responded swiftly, warning 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.
Israel has also stepped up the rhetoric against Syria, warning it could take military action if any of its advanced weapons end up in the hands of Hezbollah.
Makdissi stressed later in an e-mail 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".
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.
Assad offered 'safe exit'
Syrian spokesperson Makdissi said Damascus firmly rejected a demand by the Arab League that Assad step down.
The Arab League had called late on Sunday on Assad to "renounce power", promising he and his family would be offered "a safe exit".
Tens of thousands of Syrians have fled for neighbouring countries in recent days after violence mounted, particularly in Damascus.
Iraqi Prime Minister Nuri al-Maliki instructed the country's forces and the Red Crescent to allow in Syrian refugees, reversing an earlier decision.
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.
19 000 dead
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 aircraft 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.