Nato has given the go ahead for Patriot surface-to-air missiles to be deployed along Turkey's border with Syria, according to an official.
Turkey, which has strongly supported rebels fighting to overthrow President Bashar al Assad, said it wanted the missile interceptor systems to counter the threat of attacks from Syrian weapons, possibly carrying chemical weapons.
Nato Secretary-General Anders Fogh Rasmussen said: "We stand with Turkey in the spirit of strong solidarity.
"To anyone who would want to attack Turkey, we say, 'Don't even think about it!'"
The alliance's 28 members said the use of Patriots would be limited solely to the defensive purpose of protecting Turkey from missiles, mortar rounds and shells from Syria.
Five Turks have been killed by mortar and rocket fire from the conflict across the border.
Sky's Foreign Affairs Editor Tim Marshall, reporting from Damascus , said the deployment was a "significant moment" in the 22-month conflict.
"It requires extra Nato troops to move these Patriot missile batteries into Turkey, right up to Syrian border," he said.
"It may just be in the low hundreds but it is still a moment where hundreds of Nato troops, possibly, arrive on Syria's northern border."
Mr Rasmussen said the Patriot systems - which includes missiles, radar and other elements - would in no way support a no-fly zone over parts of Syria nor aid any offensive operation against the regime.
Germany and the Netherlands are expected to provide Turkey with several batteries of the latest PAC-3 version of the US-built Patriot air defence systems, which is designed to intercept incoming missiles.
Mr Rasmussen also joined the US, France and other Western nations in warning Mr Assad against using chemical weapons against his own citizens.
"The Nato ministers unanimously expressed grave concern about reports that the Syrian regime may be considering the use of chemical weapons," he told a news conference.
"Any such action would be completely unacceptable and a clear reach of international law."
US intelligence officials on Monday said they had detected signs the Syrian government was moving chemical weapons components around within several sites in recent days. The activities involved movement within the sites, rather than the transfer of components in or out of various sites, two of the officials said.
Syria is believed to have hundreds, if not thousands, of tons of chemical agents, including mustard gas, a blistering agent, and the more lethal nerve agents sarin and VX, experts say.
In July, Syria threatened to use chemical and biological weapons if it was attacked by a foreign attack, in what was the regime's first acknowledgement that it possessed weapons of mass destruction.
Meanwhile, at least nine children and one teacher were killed after a mortar hit a classroom in a Damascus suburb, according to state media.
The mortar reportedly hit the al-Batiha school in al-Wafideen camp, about 15 miles northeast of the capital, which houses 25,000 people displaced from the Golan Heights since the 1967 war between Syria and Israel.
The state-run Sana news agency blamed the attack on terrorists, the term the regime uses for rebels who are fighting to topple the government.
Damascus has seen some of its heaviest fighting in months in recent days and Syrian forces have begun to target rebel positions in the capital with artillery.