INCIRLIK AIR BASE, (Reuters) – U.S. Defense Secretary Leon Panetta signed an order on Friday to send two Patriot missile batteries to Turkey with 400 American personnel to operate them, in a move by NATO members to bolster Turkey’s defenses against the threat of Syrian missiles.

The order was signed shortly before Panetta arrived on an unannounced visit to Turkey to meet American troops stationed at the Incirlik Air Base, the last stop on a week-long trip that took him to Afghanistan and Kuwait.

“The purpose of this deployment is to signal very strongly that the United States, working closely with our NATO allies, is going to support the defense of Turkey, especially with potential threats emanating from Syria,” spokesman George Little said.

NATO-member Turkey has repeatedly scrambled jets along the countries’ joint frontier and responded in kind when shells from the Syrian conflict came down inside its borders, fanning fears that the civil war could spread to destabilize the region.

The widely expected U.S. move follows similar steps by Germany and the Netherlands, which also said they will send two Patriot batteries. The three countries are the only NATO nations with the most modern type of Patriots.

Little declined to say where the U.S. batteries would be located and said the systems would be deployed to Turkey for an unspecified amount of time.

“We expect them to be deployed in the coming weeks,” Little said.

NATO approved Turkey’s request for air defense batteries on December 4, in a move meant to calm its fears of coming under missile attack, possibly with chemical weapons, from Syria.

The Patriot system is designed to intercept aircraft or missiles. NATO says the measure is purely defensive, but Russia, Syria and Iran have criticized the decision, saying it increases regional instability.