BELBEK, Crimea — Russian forces pushed Saturday to complete their expulsion of the Ukrainian military from the disputed Crimean Peninsula, smashing through the gates of a base here with armored vehicles, firing weapons into the air and demanding that the cornered Ukrainian soldiers surrender.

The operation to seize the base — one of the Ukrainian military’s last strongholds on the peninsula — was larger and more dramatic than at other installations where Ukrainian forces have capitulated steadily in recent days as Russia declared its formal annexation of the region.

By evening, Russian forces were fully in control of the base here, and most of the Ukrainian troops were dispatched to their barracks and homes to pack.

The takeover came as Russia seemed to try to assure the West that its incursion into Ukraine would not expand beyond Crimea. The Kremlin agreed to allow a team of international monitors to work elsewhere in Ukraine. Fears of a further invasion had increased on Friday as Russian troops massed along Ukraine’s eastern border, and the United States issued a stern warning in response.