The expert-level technical talks could determine the specifics of nuclear and sanctions commitments by the two sides.
Firearms are the leading cause of death for US young people aged 18 to 25, surpassing drug poisoning and other causes.
Dozens of heads of state - some of whom had better relations with the pope than others - will be in attendance.
Kremlin aide says discussion focused on the 'possibility' of resuming direct negotiations between Russia and Ukraine.