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“Russia’s response will be asymmetrical, fast and tough,” Mr. Putin said last April, referring to the kinds of unconventional military action that Russia could take if adversaries threatened “our fundamental security interests.”

The current crisis was touched off by the Kremlin’s release of a series of demands that, if the U.S. and its allies agreed, would effectively restore Russia’s sphere of influence close to Soviet-era lines, before NATO expanded into Eastern Europe. It has also demanded that all U.S. nuclear weapons be withdrawn from Europe, saying it felt threatened by their presence — though the types and locations of those weapons haven’t changed in years. And it wants a stop to all Western troop rotations through former Warsaw Pact states that have since joined NATO.

It has reinforced those demands, which the U.S. calls “non-starters,” with a troop buildup near Ukraine and repeated warnings it was prepared to use unspecified “military-technical means” to defend what it considers its legitimate security interests.

In response, the Biden administration has issued warnings of financial and technological sanctions if the Kremlin should follow through with its threats, particularly in regard to Ukraine. American officials say that for all the talk about moving nuclear weapons or using asymmetrical attacks, so far the U.S. has seen little evidence.

At a White House briefing on Thursday, Jake Sullivan, Mr. Biden’s national security adviser, declined to be drawn into the question of what kind of Russian action would trigger a U.S. response — whether, for example, the U.S. would respond to a cyberattack the way it would an incursion into Ukrainian territory.

“The United States and our allies are prepared for any contingency, any eventuality,’’ he said. “We’re prepared to keep moving forward down the diplomatic path in good faith, and we’re prepared to respond to fresh acts. And beyond that, all we can do is get ready. And we are ready.”