Russia’s military staged a provocative new act of aggression Saturday, occupying a natural gas distribution center and village on a strip of Ukrainian land near the Crimean Peninsula and prompting Kiev’s Ministry of Foreign Affairs to denounce “a military invasion by Russia.”

The incident marked the first face-to-face standoff between the Ukrainian and Russian militaries outside the Crimean Peninsula, suggesting that Moscow is testing the will of Kiev amid fears of further Russian incursions in eastern and southern Ukraine. The move comes on the eve of a vote in Crimea on whether the residents there want to break away from Ukraine and join Russia.

Russian troops in four helicopter gunships and three armored combat vehicles descended on the natural gas facility near the village of Strilkove about 1:30 p.m. local time, according to Ukrainian officials. The Russians said they had seized the site out of fears it would be targeted by “terrorists,” according to a Ukrainian Defense Ministry official who spoke on the condition of anonymity.

A Ukrainian border guard spokesman, Oleg Slobodyan, said Saturday evening that 120 Russian soldiers were still occupying the site and had not agreed to Ukrainian demands to leave. No shots had been fired, he said, but added that Ukrainian military forces had mobilized to just outside the city of Henichesk, putting themselves between the Russian force and the Ukrainian mainland.