ANKARA, Turkey — Turkey has asked NATO to extend the deployment of Patriot missile batteries sent to defend the country from a possible attack from Syria because the threat remains, a foreign ministry official said on Wednesday.

Turkey sent its request to NATO Secretary-General Anders Fogh Rasmussen and it will be reviewed by Germany, the Netherlands, and the United States, which each sent two Patriot missile batteries after Ankara asked for NATO's help in beefing up security along its border with Syria.

The six missile batteries were deployed in early 2013 for one year in three southern Turkish provinces near the border.

The Turkish official told Reuters he expected Turkey's allies to approve the request.

Turkey has been one of President Bashar Assad's fiercest critics, leading calls for international intervention in the conflict, supporting the Syrian rebels and providing shelter for nearly 700,000 Syrian refugees.

The frontier has become a flashpoint in the civil war, now in its third year, with shells fired from Syrian territory frequently landing inside Turkey, drawing a response in kind from the Turkish military.