U.S. Offers Limited Initial Response to Russia as It Weighs Stiffer Sanctions

“I don’t doubt they are already calculating how to realize these plans,” Mr. Putin said.

American officials have said for months that there are no such plans, and Mr. Biden said at a January news conference that Ukraine was still years away from qualifying for NATO membership. But he has been unwilling to bend to Mr. Putin’s demand that NATO cease accepting new members, and that he provide a written, binding guarantee that Ukraine would never be part of the Western alliance.

Mr. Biden’s options for countering any military moves into Ukraine in coming days are limited. He has said repeatedly that he would not allow American forces to fight in Ukraine. But there is no guarantee that the conflict will not spill over Ukraine’s borders.

Europe was girding for refugees fleeing a modern, if corruption-ridden, democracy whose president, Volodymyr Zelensky, was in Munich on Saturday pleading his case that NATO must open its doors to his country — and thus defend it. Mr. Biden’s own national security officials have been warning American utilities, banks and other businesses to harden their networks against what they fear will be a wave of Russian-origin cyberattacks and ransomware, which they have publicly warned could be unleashed in reaction to the sanctions.

While Mr. Biden’s actions on Monday night were restrained, his administration denounced the Russian decision as a breach of the rules governing the global order.

In a statement, Jen Psaki, the White House press secretary, called Mr. Putin’s move a “blatant violation of Russia’s international commitments.”

The secretary general of the United Nations, António Guterres, called the action a violation of his organization’s charter. The United Nations Security Council held an emergency meeting late Monday night in which the United States and its allies denounced Moscow’s actions — but Russia holds a veto in that body, assuring that it could block any action.

In a joint statement, the European Commission president, Ursula von der Leyen, and the European Council president, Charles Michel, wrote that the European Union would “react with sanctions against those involved in this illegal act,” and that it “reiterates its unwavering support to Ukraine’s independence, sovereignty and territorial integrity within its internationally recognized borders.”