NATO warned Syria's president Bashar al-Assad not to use chemical weapons as it approved the deployment of Patriot interceptor missile batteries on the border of neighbouring Turkey.
The warning from NATO secretary-general Anders Fogh Rasmussen came as US government sources said Washington had information 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 Mr Assad's power base.
Syrian state media said a rebel mortar attack on a school had killed 28 students and a teacher.
International concern over Syria's intentions has been heightened by reports that its chemical weapons have been moved and could be prepared for use.
Will the US step in?
The United States could try to secure Syria's chemical arsenal by sending in special forces and staging bombing raids, but any military action would be high-risk with a chance that weapons might fall into the wrong hands, experts and former officials say. Read more
"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, adding: "We say to anyone who would want to attack Turkey - don't even think about it."
Learn more about Patriot missiles.
Turkey formally asked its NATO partners to deploy the US-made anti-missile system after a series of cross-border shellings, including one that left five civilians dead on October 3.
Germany, the Netherlands and the United States have agreed to provide the Patriot batteries, a NATO statement said.
Mr Rasmussen was at great pains to emphasise that the deployment was defensive only, and did not mean the implementation of a no-fly zone.
The French Foreign Ministry referred to "possible movements on military bases storing chemical weapons in Syria" and said the international community would react if the weapons were used.
On Monday US president Barack Obama told Mr 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 told the Reuters news agency.
One of the sources said that there was no question that the US "intelligence community" had received information pointing to "preparations" underway 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.
Patriots: The 'hit-to-kill' star missiles of the US armoury
- Patriot missiles are an anti-ballistic defence system developed by Lockheed Martin.
- Patriot stands for 'Phased Array Tracking Radar to Intercept on Target'.
- The PAC-3 is the latest generation, with 16 missiles loaded onto a single launcher compared with four of the previous PAC-2 generation.
- The latest-PAC-3 is said to take out cruise and ballistic missiles as well as aircraft.
- The PAC-3 destroys incoming targets by directly smashing into them.
- The missiles speed towards an impact point calculated before launch by a sophisticated radar and tracking system on the ground.
- But they can be re-directed once launched thanks to an on-board guidance system.
- The time from launch to point of impact is usually only between a minute and 90 seconds.