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 as the vote in Crimea begins on whether the residents there want to break away from Ukraine and join Russia.

On Sunday, the Ukrainian Defense Ministry said armed men raided a Ukrainian base under the control of Crimean self defense militias and siphoned gasoline stored there into oil tankers.

In the overnight incursion near Simferopol, about 25 men appeared with authorization from the pro-Russia Crimean government, according to Vladislav Seleznyov, a ministry spokesman. He said they filled two oil tankers that drove away, and two more arrived to be filled.

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.

In a statement, Ukraine’s Foreign Ministry demanded that the Russian side withdraw immediately and said that “Ukraine reserves the right to use all necessary measures to stop the military invasion by Russia.”

Ukraine’s Defense Ministry initially said in a statement that the Russians had left the area after a period of negotiation. But the ministry later corrected itself, saying its earlier statement was based on false information and that the Russians were still occupying the site.

It was not immediately clear whether the incursion was a temporary reconnaissance mission or a Russian attempt to secure the Ukrainian energy outpost just off the Crimean coast.

In Washington, President Obama’s top national security advisers gathered at the White House on Saturday afternoon to discuss the Ukraine crisis, and officials said Obama was being regularly updated on the situation.

Earlier in the day, Ukrainian officials reported a shootout between pro-Russian and pro-Ukrainian demonstrators that took place overnight in the eastern Ukrainian city of Kharkiv.

Two people were dead and several injured after a group of Russian separatists approached offices being used by pro-Ukrainian activists, according to Tatiana Gruzinskaya, spokeswoman for the city’s mayor.

The Russian Foreign Ministry issued a menacing statement Saturday in response to the violence. “There have been many appeals for Russia to protect peaceful civilians,” the statement said. “These appeals will be considered.”

That warning, which followed similar threats, is likely to add to fears that Russia will expand its intervention beyond Crimea and into eastern Ukraine. While Russia has blamed such clashes on right-wing Ukrainians and on the new government, Ukrainian officials placed responsibility squarely on Russia’s shoulders.

Speaking from the Ukrainian capital of Kiev, interim President Oleksandr Turchynov said that it was “Russian agents” who were “causing people to be murdered.”

As Turchynov spoke, tens of thousands of antiwar demonstrators turned out in Moscow to protest Russia’s intervention in Ukraine, marking the largest political rally in the Russian capital in more than a year.

‘Stop interfering’

Acting Interior Minister Arsen Avakov wrote on his Facebook page Saturday that it was not yet clear whether the people killed in Kharkiv were pro-Russian or pro-Ukrainian, and activists on both sides were arrested, officials said.

Ukrainian Foreign Minister Andriy Deshchytsya called on Russia to “stop interfering” in eastern Ukraine, saying that authorities here could handle issues of law and order in the region. He described the threatening statements from Russia’s Foreign Ministry as an attempt to “provoke” an escalation of the tense situation in the east.

[READ: Russia supporters in eastern Ukraine pose challenges to government]

Deshchytsya called for U.N. monitors to fan out across southern and eastern Ukraine to independently assess the situation on the ground.

He also reiterated Kiev’s position that Sunday’s referendum in Crimea would be “illegal” and would not be recognized by the international community. He emphasized the new government’s desire to avoid any military conflict and referred to the current stand off as more of a “diplomatic war.” But he added that he would travel to Brussels on Monday to meet with NATO’s secretary general to discuss “military and technical cooperation.”

Russia on Saturday blocked passage of a U.N. Security Council resolution that would have declared that Sunday’s referendum “can have no validity, and cannot form the basis for any alteration of the status of Crimea.” The veto was expected, but in a possible sign of Russian isolation, Moscow’s close ally China opted to abstain rather than join Moscow in vetoing the measure. The other 13 Security Council members supported the measure.

U.S. Secretary of State John F. Kerry on Friday warned against a “back-door annexation” of Crimea by Russia. But he conceded that six hours of talks in London with Russia’s top diplomat neither stopped the referendum nor opened a new diplomatic path for Moscow to step back from the Cold War-tinged standoff.

Tightening grip

The vote will be held under the eyes of Russian troops, who effectively took control of Crimea late last month after protesters overthrew the Ukrainian government. A Ukrainian Defense Ministry official said that a column of about 50 armored vehicles from the Russian Federation was observed Friday night moving from the city of Feodosia to Dzhankoi in northern Crimea. Ukrainian military spokesman Vladislav Seleznyov said that more Russian troops have moved in to occupy Ukrainian military bases that had been abandoned long before the crisis began.

Moscow’s tightening grip on Crimea and the gathering of Russian troops along the countries’ border have unnerved Ukrainians and left the country’s fledgling government concerned about further Russian military action. Kerry said the United States was “deeply concerned” about those deployments.

[READ: Ahead of Crimea vote, Ukraine officers express pro-Russia sentiments]

If Russia does move to annex Crimea after Sunday’s vote, it could trigger the harshest sanctions levied against Moscow by the United States and European powers since the Cold War.

The European Union is expected to impose travel bans and asset freezes Monday on Russians accused of complicity in Moscow’s military incursion and the intimidation of Crimea. The E.U. on Friday identified more than 120 individuals as potential sanctions targets.

The White House announced Friday that Vice President Biden will travel to Poland and Lithuania next week to discuss Ukraine and other issues with regional leaders.

Englund reported from Moscow. Pamela Constable in Simferopol, Carol Morello in Sevastopol, Kathy Lally in Moscow and Griff Witte in London contributed to this report.