Nato has told Syrian President Bashar al-Assad that any use of chemical weapons in his fight against encroaching rebel forces would be met by an immediate international response.
The warning from Nato Secretary-General Anders Fogh Rasmussen came as US government sources said Washington had information that Syria was making what could be seen as preparations to use its chemical arsenal.
Syrian forces meanwhile bombarded rebel districts near Damascus in a sustained counter-attack to stem rebel gains around Assad's power base as the insurgency may be entering a decisive phase.
International concern over Syria's intentions has been heightened by reports that its chemical weapons have been moved and could be prepared for use.
"The possible use of chemical weapons would be completely unacceptable for the whole international community and if anybody resorts to these terrible weapons I would expect an immediate reaction from the international community," Mr Rasmussen told reporters at the start of a meeting of alliance foreign ministers in Brussels.
The chemical threat made it urgent for the alliance to send Patriot anti-missile missiles to Turkey, he said.
The French foreign ministry referred to "possible movements on military bases storing chemical weapons in Syria" and said the international community would react if they were used.
Britain has told the Syrian government that any use of chemical weapons would have "serious consequences", foreign secretary William Hague said.
US president Barack Obama yesterday told Dr Assad not to use chemical weapons, without saying how the United States might respond. The foreign ministry in Damascus said it would never use such weapons against Syrians.
The US has collected what has been described as highly classified intelligence information demonstrating that Syria is making what could be construed as preparations to use elements of its extensive chemical weapons arsenal, two US government sources briefed on the issue said.
One of the sources said that there was no question that the US "intelligence community" had received information pointing to "preparations" under way in Syria related to chemical weapons. The source declined to specify what kind of preparations had been reported, or how close the intelligence indicated the Syrians were to deploying or even using the weapons.
Western military experts say Syria has four suspected chemical weapons sites, and it can produce chemical weapons agents including mustard gas and sarin, and possibly also VX nerve agent. The CIA has estimated that Syria possesses several hundred litres of chemical weapons and produces hundreds of tonnes of agents annually.
The fighting around Damascus has led foreign airlines to suspend flights and prompted the United Nations and European Union to reduce their presence in the capital, adding to a sense that the fight is closing in.
The army fightback came a day after the Syrian foreign ministry spokesman was reported to have defected in a potentially embarrassing blow to the government.
The Syrian Observatory for Human Rights said 200 people were killed across Syria on Monday, more than 60 of them around Damascus.
Dr Assad's forces bombarded districts to the south-east of the capital today, near to the international airport, and in the rebel bastion of Daraya to the south-west.
Opposition footage posted on the internet showed a multiple rocket launcher fire 20 rockets, which activists said was filmed at the Mezze military airport in Damascus.
Reuters could not independently verify the footage due to the government's severe reporting restrictions.
In central Damascus, shielded for many months from the full force of a civil war in which 40,000 people have been killed, one resident reported hearing several loud explosions.
"I have heard four or five thunderous blows. It could be barrel bombs," she said, referring to makeshift bombs which activists say Dr Assad's forces have dropped from helicopters on rebel-dominated areas.
Reuters