The Durg class corvettes of the Indian Navy were customized variants of the Soviet Navy Nanuchka class corvettes. Three vessels of this class served in the Indian Navy, where they formed the 21st Missile Vessel Squadron (K21). Durg class vessels take their names from famous historical forts in India. The Durg class is also known as the Nanuchka II class. Some design improvements made for the Durg class were incorporated into the Soviet Nanuchka III class, commissioned into the Soviet Navy in the 1980s, some of which are still in service with the Russian Navy as of 2009.