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KIEV, Ukraine – At least 80 Russian troops landed by helicopter Saturday to seize a natural gas terminal just beyond the regional border of Crimea, the Ukrainian government said. The action was Russia’s most provocative since its forces took over Crimea two weeks ago.

The latest troop advance comes one day before Crimea is to vote on whether to secede from Ukraine and is testing Ukrainian leaders’ resolve to engage Russia’s much more powerful military if it moved beyond Crimea.

By late afternoon, Ukranian troops were stationed outside the gas plant, which is on a slender sand bar to the east of the Crimean Peninsula, according to Unian, a Ukrainian news service that spoke to local police. The news agency did not say if shots had been fired.

The Ukrainian foreign ministry issued a statement saying Ukraine “reserves the right to use all necessary measures” to stop what it called “the military invasion by Russia.”

Ukrainians have feared for some time that as Russia tightened its control over Crimea, it would also try to secure key infrastructure supporting the peninsula that lie outside its administrative borders, including a reservoir of fresh water in Ukraine’s Kherson region.

Unian reported initially that helicopters had landed near the village of Strelkovoye on the Arabatskaya Spit, in the Azov Sea that links Crimea to the mainland.

The Ukrainian foreign ministry statement said about 80 troops landed on the spit, supported by four helicopters and three armored vehicles. The Ukrainian ministry of defense said it also had scrambled aircraft without specifying what type.

A Pentagon spokeswoman said Saturday afternoon that U.S. officials had not confirmed the reported incursion and had no comment.

The Russian move appeared to fit the pattern of deployment on Crimea: Unian cited local residents saying soldiers without identifying insignia had landed near the gas terminal but their origin was hardly in doubt, as they had landed in helicopters with Russia’s red-star tail art.

Even before the incursion, Ukraine’s acting president, Oleksandr V. Turchynov, warned Saturday that “the situation is very dangerous. I am not exaggerating. There is a real danger of an invasion of Ukrainian territory.” He called on Parliament to hold an extraordinary session Monday morning to authorize emergency funding to the ministry of defense.

Ukrainian officials were especially anxious because of reports of street fighting breaking out in cities in the eastern provinces of Ukraine between protesters who are pro-Russian and those who support the Ukrainian government in Kiev.

Russian officials have said they stood ready to enter the Ukrainian mainland if they felt ethnic Russian citizens were in danger.

On Saturday, Russia’s foreign ministry reiterated that.

“Russia has received many calls to protect peaceful citizens,” the foreign ministry said in a statement. “These requests are being considered.”