With Russia refusing to pull back 100,000 troops poised on Ukraine's borders, Pentagon spokesman John Kirby said 1,000 US soldiers now in Germany would deploy to Romania, and another 2,000 in the United States would be sent to Germany and Poland.
"The current situation demands that we reinforce the deterrent and defensive posture on NATO's eastern flank," Kirby said.
"President Biden has been clear, the United States will respond to the growing threat to Europe's security and stability."
Kirby stressed that the move was to demonstrate US commitment to the NATO alliance and that no American troops would be sent to fight in Ukraine, which is not a NATO member.
Russian President Vladimir Putin has demanded guarantees that Ukraine will not join the alliance, and has implicitly threatened the former Soviet state with the massive military buildup along the two countries' frontier.
Russia also wants NATO and the United States to foreswear the deployment of missile systems near Russia's borders and to pull back NATO forces in eastern Europe.
Russia Claims Chinese Support
The US announcement came one day after Putin, in his first major remarks on the crisis in weeks, suggested Washington was using Kyiv as an instrument to potentially drag Moscow into a war.
The West is seeking to contain Russia, he said, and "Ukraine itself is just a tool to achieve this goal."
Putin left the door open to talks, saying he was studying Western proposals and that he hoped that "in the end, we will find a solution."
The Kremlin said Wednesday it had Beijing's support in the standoff, and that would emerge when Putin meets Chinese leader Xi Jinping in Beijing on Friday.
"China supports Russia's demands for security guarantees," the Kremlin's top foreign policy adviser Yury Ushakov told reporters.
Both sides continued to weigh proposals exchanged for de-escalating the increasingly tense situation, but there was no sign of compromise Wednesday.
Dutch Prime Minister Mark Rutte was the latest NATO leader to visit Kyiv in a show of support for Ukraine, where he met President Volodymyr Zelensky.
After the talks, Zelensky said Ukraine was focused "only on peace", but insisted it has the right to defend itself.
"It is essential for dialogue to continue between Russia and the United States, between Russia and NATO," said Rutte.
NATO welcomed the new US troop deployment, with Secretary-General Jens Stoltenberg saying "NATO will do whatever is necessary to protect and defend all allies."
Citing leaked documents, the Spanish newspaper El Pais reported that Western proposals to Russia include arms control and trust-building measures to defuse the situation.
While a guarantee against Ukraine joining NATO was not offered, the documents proposed "reciprocal commitments by both the United States and Russia to refrain from deploying offensive ground-launched missile systems and permanent forces with a combat mission in the territory of Ukraine," El Pais reported.
"We did not make this document public," Kirby said.
Nevertheless, he said, it confirms that "NATO and its partners are unified in their resolve and open to constructive and serious diplomacy."
Russia 'Adding Forces'
But Kirby said Moscow has shown "no signs of being interested or willing to de-escalate the tensions."
"Mr. Putin continues to add forces, combined arms, offensive capabilities, even over just the last 24 hours. He continues to add in western Russia and Belarus and ... the Mediterranean and the North Atlantic."
Tensions have been further aggravated by plans for joint military exercises between Russia and neighboring Belarus, where Washington claims Moscow is preparing to send 30,000 troops.
Video footage released by the Russian defense ministry on Wednesday showed tanks speeding across snowy fields in Belarus and combat helicopters flying overhead as units from both countries practiced ahead of the February 10-20 drills.