Operation Ring, known in Azerbaijan as Operation Chaykand was the codename for the May 1991 military operation conducted by the Soviet Army, Internal Troops of the Ministry of Internal Affairs (MVD) of the USSR and OMON units of the Azerbaijan SSR in the Khanlar and Shahumyan districts of the Azerbaijani SSR, the Shusha, Martakert and Hadrut districts of the Nagorno-Karabakh Autonomous Oblast, and along the eastern border of the Armenian SSR in the districts of Goris, Noyemberyan, Ijevan and Shamshadin. Officially dubbed a "passport checking operation," the ostensible goal of the operation was to disarm "illegal armed formations" in and around Nagorno-Karabakh, referring to irregular Armenian military detachments that had been operating in the area. The operation involved the use of ground troops accompanied by a complement of military vehicles, artillery and helicopter gunships to be used to root out the self-described Armenian fedayeen.

OMON is a system of special police units within the National Guard of Russia. It previously operated within the structures of the Soviet and Russian Ministries of Internal Affairs (MVD). Originating as the special forces unit of the Soviet Militsiya in 1988, it has played major roles in several armed conflicts during and following the 1991 dissolution of the Soviet Union.