ISTANBUL - Turkey accused Russia of a "provocation" on Sunday after a serviceman on the deck of a Russian naval ship allegedly held a rocket launcher on his shoulder while the vessel passed through Istanbul.Relations have deteriorated sharply since Turkey last week became the first NATO member in more than half a century to down a Russian plane, which it said had violated its airspace while flying sorties over Syria. The pilot was killed.The NTV news channel broadcast photographs that it said showed a serviceman brandishing a rocket launcher on the deck of the landing ship Caesar Kunikov as it passed on Saturday through the Bosphorus Strait, which bisects the city of Istanbul. It said the ship was believed to be en route to Syria."For a Russian soldier to display a rocket launcher or something similar while passing on a Russian warship is a provocation," Foreign Minister Mevlut Cavusoglu told reporters, according to the Hurriyet news site. "If we perceive a threatening situation, we will give the necessary response."