WASHINGTON — The United States plans to send two Patriot missile batteries and about 400 military personnel to Turkey to defend against a possible Syrian missile attack, two allied officials said Thursday.

The American batteries will be part of a broader push to beef up Turkey’s defenses that will also include the deployment of four other Patriot batteries — two from Germany and two from the Netherlands.

All six batteries will be under NATO’s command and control, scheduled to be operational by the end of January.

Turkey, which has been supporting the Syrian opposition to President Bashar al-Assad, has been worried it is vulnerable to Syrian missiles, including Scuds that might be tipped with chemical weapons. Those concerns were heightened by reports of increased activity at some of Syria’s chemical sites, though Defense Secretary Leon E. Panetta said this week that intelligence about chemical weapons activity in Syria had “leveled off.”