NATO has given the go-ahead to stationing Patriot surface-to-air missiles in Turkey to protect the country from any spillover of the civil war in neighbouring Syria.
Russia, Syria and Iran have criticised Turkey's request to the alliance to send the Patriots which can be used to intercept missiles.
Turkey asked NATO for the missiles in November after weeks of talks with allies about how to shore up security on its 900km border.
It has repeatedly scrambled fighter jets along the frontier and responded in kind to stray Syrian shells flying into its territory.
A major player in supporting Syria's opposition and planning for the post-conflict era, Turkey is worried about Syria's chemical weapons, the refugee crisis along its border, and what it says is Syrian support for Kurdish militants on its own soil.
Turkey made similar calls for military support during the two Gulf Wars, when NATO deployed surface-to-air missiles on its soil in 1991 and 2003.
NATO warns Syria against using chemical weapons
NATO has told Syrian President Bashar al-Assad that any use of chemical weapons in his fight against rebel forces would be met by an immediate international response.
The warning from NATO Secretary-General Anders Fogh Rasmussen came as Syrian forces bombarded rebel districts near Damascus in a sustained counter-attack to stem rebel gains around 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.
"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 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.
Yesterday, US President Barack Obama told 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.
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 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 yesterday, more than 60 of them around Damascus.
President Assad's forces bombarded districts to the south-east of the capital on Tuesday, 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.
The footage could not be independently verified due to the government's severe reporting restrictions.