Siberia is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed part of the sovereign territory of Russia and its predecessor states since the centuries-long conquest of Siberia, which began with the fall of the Khanate of Sibir in the late 16th century and concluded with the annexation of Chukotka in 1778. Siberia is vast and sparsely populated, covering an area of over 13.1 million square kilometres (5,100,000 sq mi), but home to roughly a quarter of Russia's population. Novosibirsk and Omsk are the largest cities in the area.