Nato foreign ministers will agree to send Patriot missiles to beef up Turkey's air defences and calm Turkey's fears that it could come under missile attack, possibly with chemical weapons, from Syria, diplomats say.
Turkey last month asked Nato for Patriots, which can be used to intercept missiles and planes, after weeks of talks with allies about how to shore up security on its 900km border with Syria, which is immersed in civil war.
Foreign ministers from the 28-nation alliance are expected to give their backing to the move when they meet in Brussels for a twice-yearly meeting, sending a strong signal they stand behind their ally Turkey.
"There will be a decision and probably a statement from the ministers themselves," a Nato diplomat told reporters.
The move follows media reports, citing European and US officials, that Syria's chemical weapons had been moved and could be prepared for use in response to dramatic gains by rebels fighting to topple President Bashar al-Assad.
Syria said on Monday it would not use chemical weapons against its own people after the United States warned it could take action against any such escalation.
"Turkey's request, when it came to Patriots, was to augment its air defences with the capacity to deal with the threat of ballistic missiles and particularly the threat of ballistic missiles potentially armed with chemical warheads," another Nato diplomat, speaking on condition of anonymity, said.
The diplomat said the ministers would simply agree on Tuesday to "augment Turkey's air defences".
"The decision on whether or not to deploy Patriots, and for how long is, like all Nato decisions when it comes to the deployment of military forces, a national one," he said.
The United States, Germany and the Netherlands are the countries expected to supply Turkey with Patriots. Germany and the Netherlands may need parliamentary approval to send the missiles and deployment could take weeks.
The first diplomat said that Nato's Supreme Allied Commander Europe, US Admiral James Stavridis, had the power to deploy Nato's own fleet of Awacs surveillance planes if he judged it necessary to counter a specific threat and would not need ministerial approval.
However, there is no immediate plan for him to do so in the case of Turkey.
The Nato foreign ministers will take the Patriot decision immediately after they hold talks in Brussels with Russian Foreign Minister Sergei Lavrov, who is expected to repeat Moscow's opposition to the move.
Russia, which has a fractious relationship with the military alliance, has been at odds with Nato over how to end the Syrian conflict and has vetoed UN resolutions aimed at pressuring Assad to step down.
Turkey repeatedly has scrambled jets along the countries' joint border and responded in kind when shells from the conflict came down inside its borders, underlining fears Syria's civil war could spread to destabilise the region.
A senior State Department official accompanying US Secretary of State Hillary Clinton to Europe for the Nato talks, said he did not expect final details this week on the number of missiles that would be deployed, where or for how long, as site surveys were still going on.
He said the deployment would not be part of "an inexorable move towards a no-fly zone" over Syria, of the sort Nato mounted to defend anti-government rebels in Libya who toppled Muammar Gaddafi last year.
In Prague, Clinton reiterated a warning against any attempt by the Syrian government to use its chemical weapons stockpile against the rebels, calling this a "red line" that would prompt US action.
"I am not going to telegraph any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people, but suffice to say, we are certainly planning to take action if that eventuality were to occur," she said.
The second day of the Nato meeting on Wednesday will focus on progress in the alliance's efforts to gradually hand over security control to Afghan forces as it prepares to wind down its combat operations by the end of 2014.
US officials say Clinton will seek to encourage what Washington sees as an improving mood between Pakistan and its neighbour Afghanistan.