Nato has approved the deployment of Patriot anti-missile batteries along Turkey’s border with Syria.
The long-expected move emerged from a meeting of Nato foreign ministers in Brussels, and amid growing fears that Syria could use chemical weapons.
Nato’s Secretary General Anders Fogh Rasmussen and the US have said this would be “completely unacceptable”.
Syria has insisted it will not use chemical weapons against its own people, “in any circumstances”.
The meeting of the 28-member Western military alliance’s foreign ministers in Brussels follows a request from Turkey to boost its defences along the border.
As he emerged from the meeting, Mr Rasmussen tweeted: “Nato has agreed to augment Turkey’s air defence capabilities. We are determined to defend Turkey’s people and territory.”
Nato officials have previously made clear such a move would be purely defensive.
Ankara’s request came after intelligence assessments that Damascus was contemplating using ballistic missiles, potentially armed with chemical warheads, reports say.
The BBC’s diplomatic correspondent James Robbins says Nato’s move is an expression of solidarity with Turkey, and a signal to Syria’s President Bashar al-Assad he must not widen the war against his own people beyond Syria’s borders.
Syria is believed to hold chemical weapons – including mustard gas and sarin, a highly toxic nerve agent – at dozens of sites around the country.
The CIA has said those weapons “can be delivered by aircraft, ballistic missile and artillery rockets”.
A Nato team has already visited a number of sites in Turkey in preparation for the deployment of Patriot batteries, which could be used to shoot down any Syrian missiles or warplanes that stray over the border.
But analysts say any deployment – possibly supplied by the US, Germany or the Netherlands – could take weeks.
Speaking ahead of the meeting, Mr Rasmussen also said that any use of chemical weapons by Syria “would be completely unacceptable for the whole international community” and prompt “an immediate reaction”.
President Obama has previously warned President Assad he would face “consequences” if he used chemical weapons against his people.
One unnamed US official told the New York Times on Monday that the level of concern in Washington was such that contingency plans were being prepared.
Activists say more than 40,000 people have been killed since the uprising against President Bashar al-Assad began in March 2011. Hundreds of thousands of people have fled the country and another 2.5 million are internally displaced.
Syrian opposition fighters have reportedly made dramatic gains recently, and several government mortar shells – aimed at rebel targets close to the border – have landed across its 900-km (560-mile) border with Turkey.
Although the head of the Arab League Nabil al-Arabi told AFP on Monday that the Syrian government could fall “any time”, it still holds the capital, parts of the second city Aleppo, and other centres.