SIMFEROPOL, Crimea — Several Russian military helicopters landed on March 15 in southern Kherson Oblast near Kremlin-occupied Crimea, bringing dozens of commandoes and seizing a natural gas plant.

The moves heightened tension on the eve of a Crimean referendum, which Ukraine and the West consider illegal. Russian troops unexpectedly invaded the Crimean peninsula on Feb. 27 and in the following days Russian President Vladimir Putin said that he might order the use of military forces in Ukraine.