KIEV, Ukraine — Russia appeared on the brink Friday of intensifying and expanding its invasion of Ukraine, a move Ukrainian officials all but admit they would be powerless to stop.

Russian officials made statements about eastern Ukraine that were eerily similar to those they made in February about Crimea, shortly before invading and occupying the Black Sea peninsula.

Ukrainians spoke of a “Plan B” to deal with the Russians, but Prime Minister Arseniy Yatsenyuk cautioned that while there have been discussions with NATO on a technical level, “We must clearly understand that now it is the sole responsibility of Ukraine.”

Coming during a week when the Ukrainian parliament admitted the nation can call on only 6,000 battle-ready troops to defend its borders against a far better-equipped Russian force near the eastern border that is thought to number more than 200,000, Ukrainians appear not to be holding out much hope.

A Ukrainian officer reached Friday at one of a handful of Ukrainian military bases in Crimea still in Ukrainian hands said he and his fellow officers have asked officials in Kiev repeatedly for instructions on what to do but have yet to receive a response. The officer spoke on condition of anonymity because he fears for his family’s safety.

The heightening of tensions comes just two days before a referendum in Crimea in which residents are expected to vote to secede from Ukraine and ask Russia to take them in.

Ukrainian television reported that Russian troops in Crimea, known widely here as “little green army men” because their uniforms do not carry identifying insignia, said Friday that their real job begins after the referendum, when any remaining Ukrainian troops will be classified as bandits and dealt with.

Signs that Russia was considering an expansion of its presence in Ukraine came after a stabbing death during a riot between pro-Russian and pro-Ukrainian demonstrators in Donetsk, the industrial capital of eastern Ukraine.

In a mocking statement that referred to the government in Kiev as “those who call themselves the Ukrainian authorities,” the Russian Foreign Ministry said that “the Kiev authorities are not in control of the situation in the country.”

“Russia recognizes its responsibility for the lives of countrymen and fellow citizens in Ukraine and reserves the right to take people under its protection,” the statement said in wording similar to that of Russian officials before their troops moved into Crimea.

Russia repeatedly has denied that the troops now almost completely occupying Crimea are Russian, saying that they are instead Crimean militiamen seeking to defend their homeland. But the troops have acknowledged to their Ukrainian counterparts that they are Russian troops. And while Russian license plates have vanished from most of their vehicles, on Wednesday a long convoy of green military vehicles included one with Russian plates.

Russian forces appear to have captured most Ukrainian military bases in Crimea. The few bases that remain under Ukrainian control are surrounded. At one, the home of an elite Ukrainian unit that fought in both Kosovo and Iraq, a captain reported Friday that an officer from the camp was beaten by unknown assailants when he stepped outside the gates.