The United States said Tuesday that Russia's move into eastern Ukraine amounts to the "beginning of an invasion" and warned that "severe" sanctions would be announced shortly.

"We think this is, yes, the beginning of an invasion, Russia's latest invasion into Ukraine, and you're already seeing the beginning of our response, that we said will be swift and severe," deputy national security advisor Jonathan Finer told CNN.

The White House also welcomed Germany's decision to halt the mammoth Nord Stream 2 pipeline project meant to deliver Russian natural gas to Europe.

President Joe Biden "made clear that if Russia invaded Ukraine, we would act with Germany to ensure Nord Stream 2 does not move forward... We will be following up with our own measures today," Press Secretary Jen Psaki tweeted.

Russian President Vladimir Putin sharply escalated the crisis in eastern Europe on Monday when he announced recognition of independence for two separatist enclaves that Moscow supports in Ukraine -- and said Russia's military would be responsible for what he called "peacekeeping."

It was not immediately clear what the scope and timing of Russian "peacekeeping" troop movements would be and, crucially, whether Russia will now openly support the separatists in their goal to seize even more Ukrainian territory across the Donbas region.

Biden immediately reacted by imposing economic sanctions on the two enclaves.

But while the United States and other Western allies condemned a violation of pro-Western Ukraine's territorial integrity, Washington was initially cautious about whether to characterize Putin's move is an invasion, thereby triggering much wider Western sanctions against Russia itself.

"We are going to assess what Russia's done," a US official told reporters, stressing that Russian forces have already been deployed covertly in the separatist areas for eight years.

"Russian troops moving into Donbas would not be a new step," he said.

"We'll continue to pursue diplomacy until the tanks roll."

Later Monday a White House spokesperson first revealed that new sanctions would be announced, indicating that the position was hardening.

Beyond Nord Stream 2

Secretary of State Antony Blinken slammed Russia's recognition of the separatist areas as a sign Putin had no interest in negotiating, saying it "directly contradicts Russia's claimed commitment to diplomacy, and is a clear attack on Ukraine's sovereignty."

Putin's announcement also sparked intense phone diplomacy between Washington, European capitals and Ukraine as the United States tries to maintain unity among dozens of partners over how to respond to Russia, which supplies much of the European Union's energy supplies.

After announcing a stop to the near-completed Nord Stream 2 pipeline project, German Chancellor Olaf Scholz warned Russia "there are also other sanctions that we can introduce if further measures are taken."

On Friday, the deputy US national security advisor for international economics, Daleep Singh, warned that the full set of sanctions under preparation would turn Russia into an international "pariah."