ANKARA, Turkey — A second pair of Patriot missile batteries being sent by NATO countries to defend Turkey against possible attack from Syria are now operational, a German security official said on Tuesday.

The United States, Germany, and the Netherlands each committed to sending two batteries and up to 400 soldiers to operate them after Ankara asked for help to bolster its air defenses against possible missile attack from Syria.

The frontier has become a flashpoint in the 22-month insurgency against President Bashar al-Assad, with Syrian government shells frequently landing inside Turkish territory, drawing a response in kind from Ankara's military.

The two German batteries, which have been deployed around the Turkish city of Kahramanmaras some 100 km (60 miles) from the Syrian border, were in position and ready to use as of late Monday, the German security official said.

The first pair of batteries, sent by the Netherlands, went operational on Saturday around the city of Adana, southwest of Kahramanmaras. The final U.S. Patriots are expected to arrive in Turkey on Wednesday and go active in the coming days.

The batteries are being stationed around three southeastern Turkish cities and NATO says they will protect 3.5 million Turks from missile attack. Patriots are capable of shooting down hostile missiles in mid-air.

Tensions have increased in recent weeks after NATO said it had detected launches of short-range ballistic missiles inside Syria, several of which have landed close to the Turkish border. Turkey has scrambled warplanes along the frontier, fanning fears the war could spread and further destabilize the region.

Syria has called the deployment of the batteries "provocative" while Iran and Russia, which have supported Syria throughout the uprising, have criticized NATO's decision, saying the Patriots would intensify the conflict.

Turkey and NATO have strongly denied the Patriot missiles are a precursor to a no-fly zone that Syrian rebels have been requesting to help them hold territory against a government with overwhelming firepower from the air.

All six Patriot batteries will be connected directly to allied air command in Ramstein, Germany.