Russian MI-28 attack helicopters fly during the Army-2015 show at a shooting range in Alabino, outside of Moscow, Russia, on Tuesday, June 16.

The United States has identified a small number of Russian helicopters at a Syrian airfield, U.S. 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 U.S. 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 al-Assad.

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The Pentagon has said Russia’s movement of equipment and personnel suggest preparations to establish an air operations hub in Syria, where a U.S.-led coalition is carrying out airstrikes against ISIS extremists.

Moscow and Washington have said their enemy is ISIS, whose extremist fighters control part of Syria.

Russia supports Assad’s government, while the U.S. has said Assad’s presence is fueling the 4-1/2 year civil war in Syria.