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"To the Turkish people we say: We are determined to defend you and your territory. To anyone who would want to attack Turkey we say: Don't even think about it," NATO Secretary-General Anders Fogh Rasmussen said after the 28-nation alliance foreign ministers took the decision at a meeting in Brussels.
NATO-member Turkey repeatedly has scrambled jets along the countries' joint frontier and responded in kind when shells from the Syrian conflict came down inside its borders, fuelling fears that the civil war could spread to destabilise the region.
It quickly welcomed the move. "This decision is important, as it demonstrates and re-confirms allied solidarity and unity in practical terms," Turkey's Foreign Ministry said in a statement.
Germany, the Netherlands and the United States plan to provide Patriot missile batteries, the ministers said, but they gave no details on numbers. Deployment is expected to take several weeks, given the need for approval by national parliaments and the logistics of delivering the missiles.
NATO ministers unanimously expressed "grave concern" about reports that the Syrian government may be prepared to use its chemical weapons, Rasmussen said. He warned earlier that any use of chemical weapons by Syria would prompt an immediate international response.
Turkey wants to reinforce its air defences to deal with the threat of ballistic missiles from Syria, particularly a potential chemical weapons' threat, a NATO diplomat said, speaking on condition of anonymity.
German Foreign Minister Guido Westerwelle said he expected the German parliament would approve Patriot deployment.
"I have already had several discussions with different parties and my impression is that the placement of defence missiles to protect Turkey in Turkey will get a broad parliamentary majority," he told reporters in Brussels.
NATO says the measure is purely defensive, but Russia, Syria and Iran have criticised the decision, saying it increases regional instability.
After talks with NATO foreign ministers in Brussels, Russian Foreign Minister Sergei Lavrov repeated Russian concerns that the Syrian conflict was becoming increasingly militarised and accused NATO of over-reacting.
"Yes, there were artillery strikes but we believe that they were not intentional," he told a news conference.
Russia, which has a fractious relationship with the military alliance, has been at odds with NATO over how to end the war and has vetoed U.N. resolutions aimed at pressuring Syrian President Bashar al-Assad to step down.
Rasmussen said Patriot missiles would be effective as interceptors, whether attacking missiles carry chemical weapons or conventional warheads.
Asked whether blowing-up a chemical warhead in mid-air could contaminate a wide area, a NATO official said: "Any damage caused by a chemical missile attack that hits its target would be much greater than any possible damage from an intercept."
Britain has told the Syrian government that any use of chemical weapons would have "serious consequences", British Foreign Secretary William Hague said.
U.S. President Barack Obama told Assad on Monday not to use chemical weapons, without saying how the United States might respond.
The United States 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 U.S. government sources briefed on the issue said.
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.
Syria said on Monday it would not use chemical weapons against its own people and Russia's Lavrov was sceptical about the media reports that Syria was willing to use its stockpile.
"It is not the first time that the messages appear ... that Syrian authorities are transferring their stockpiles of chemical weapons to other places or that they are willing to apply these weapons," he said, according to an interpreter.
Moscow checked such reports, but "every time we get confirmation that nothing of this sort is being prepared," he said.
NATO officials have said repeatedly the alliance has no intention of getting involved in Syria's civil war, but they are concerned about the situation on its border with Turkey.