ISTANBUL — Members of a Turkish nationalist youth group assaulted three visiting American sailors in Istanbul on Wednesday, hurling balloons filled with red paint at them, putting white sacks over their heads and calling them murderers.

The Turkish authorities later arrested 12 people in connection with the assault, which the American Embassy in Ankara described as “appalling.”

A video posted online by the group, the Turkish Youth Union, or T.G.B., shows a dozen of its members staging an anti-American protest in the touristy Eminonu neighborhood and attacking the sailors, who were not in uniform, in broad daylight.

The sailors had just disembarked from the guided missile destroyer Ross, docked in Istanbul for a few days. They escaped and returned to the ship, and all shore leave for the crew was canceled.