BRUSSELS, Belgium - NATO agreed Tuesday to send new American-made air defenses to Turkey's volatile southern border with Syria, a boost to an alliance member on the front lines of the civil war and a potential backstop for wider U.S. or NATO air operations if Syria deteriorates further.

The military alliance's approval of Patriot antimissile batteries represents NATO's first significant military involvement in the 20-month-long crisis, even if it falls well short of rebel demands for help.

NATO and U.S. officials insisted that the system is entirely devoted to defending Turkey and is not a precursor to military intervention in Syria. The Patriots would provide no protection for Syrian civilians or rebels fighting to unseat President Bashar al-Assad.

However, the system, likely to deploy early next year, could be repurposed as part of a wider air campaign or to provide air cover for action in Syria should NATO change its mind. Military experts said Patriots are as effective against aircraft as they are against missiles, and deploying the system at the border could be instrumental in quickly carving out a 25-mile buffer zone.

"Turkey has asked for NATO's support, and we stand with Turkey in the spirit of strong solidarity," NATO Secretary-General Anders Fogh Rasmussen said following approval by the 28-member alliance at a meeting in Brussels. "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.' "