Nato joined Washington in delivering a blunt warning to Damascus yesterday against any use of chemical weapons, as the alliance readied to approve a Turkish request for missiles to protect its border with Syria.
In the face of deteriorating security, the United Nations on Monday suspended operations in Syria and said it would pull out non-essential staff, while the European Union reduced its activities in Damascus to a minimum.
"The possible use of chemical weapons would be completely unacceptable to the whole international community and and I would expect an immediate reaction from the international community," Nato head Anders Fogh Rasmussen said.
Syria's chemical weapons stockpiles were "a matter of great concern," he said, adding: "This is also the reason why it is a matter of urgency to ensure effective defence and protection of our ally Turkey."
Turkey's request for US-made surface-to-air Patriot missiles to be deployed on its border is worrying Russia, but both Nato and Ankara insist they would be purely defensive.
Military sources in Turkey have said Nato is considering the deployment of up to six Patriot batteries and some 300-400 foreign troops to operate them.
The Patriot, designed mainly to bring down missiles but effective also against aircraft, would likely be supplied by Germany, The Netherlands or the United States.
US President Barack Obama on Monday told Syrian President Bashar al-Assad not to use chemical weapons against his own people, in a new warning as the conflict approaches the 21-month mark with more than 41,000 people killed.
"I want to make it absolutely clear to Assad and those under his command, the world is watching, the use of chemical weapons is and would be totally unacceptable," Obama said.
"If you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable."
The Syrian government, fighting to prevent the capital Damascus from falling to rebel forces, on Monday reiterated it would never resort to chemical weapons.
But a US official told AFP that Syria had begun mixing chemicals that could be used to make sarin, a deadly nerve agent, while CNN reported Damascus could use the gas in a limited artillery attack on advancing rebels.
On the ground, the Syrian army yesterday blasted a string of rebel zones on the eastern and southwestern outskirts of Damascus.
Against this backdrop, Syria and Turkey's request for help to boost its defence is dominating the two-day Nato meeting in Brussels.
Russian President Vladimir Putin warned Monday that any deployment of the Patriots would only add to tensions and possibly widen the conflict.