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 said the ministers had "unanimously expressed grave concerns" about the use of chemical weapons.
Syria has said it would never use such weapons against its own people.
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.
In a statement, Nato said it had "agreed to augment Turkey's air defence capabilities in order to defend the population and territory of Turkey and to contribute to the de-escalation of the crisis along the alliance's border".
Recent intelligence assessments have indicated Damascus is contemplating using ballistic missiles, potentially armed with chemical warheads.
Speaking after the meeting, Mr Rasmussen told reporters that the foreign ministers had "unanimously expressed grave concerns" about the reports, saying: "Any such action would be completely unacceptable and a clear breach of international law."
He would not give further details on the deployment, but said it would ensure effective protection of Turkey against any missile attack, whether carrying chemical weapons or not.
Nato officials have previously made clear such a move would be purely defensive.
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".
President Obama has previously warned President Assad he would face "consequences" if he used chemical weapons against his people.
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.
Activists say more than 40,000 people have been killed since the uprising against President 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.
In other developments:
- A teacher and at least 28 students were killed when shelling hit their school inside the Wafideen refugee camp outside Damascus on Tuesday - state media said it was a rebel mortar attack but rebels blamed government troops.
- A journalist working for a state-run newspaper was killed near his home in Damascus, said state media.
- The UN said on Monday it was pulling "all non-essential international staff" out of Syria, with as many as 25 out of 100 international staff expected to leave this week.
- The EU has withdrawn its mission altogether - the ambassador and head of delegation Vassilis Bontosoglou left Damascus with his six remaining international staff members on Tuesday morning.
- Syrian foreign ministry spokesman Jihad Makdissi is said to have fled the country, amid reports he has been dismissed, ostensibly for making statements out of line with government policy.