WASHINGTON - The United States has identified a small number of Russian helicopters at a Syrian airfield, US officials said on Wednesday, the latest addition to what Washington believes is a significant Russian military buildup in the country.One of the officials said four helicopters had been identified, including helicopter gunships, although it was not clear when the Russian helicopters had arrived there. The officials spoke to Reuters on condition of anonymity.Reuters has previously reported on US assessments that Russia has sent about 200 naval infantry forces, battle tanks, artillery and other equipment to an airfield near Latakia, a stronghold of Syrian President Bashar Assad.