WASHINGTON — At a news conference a few days ago, Secretary of State Antony J. Blinken was asked whether the Biden administration had a “red line” in Ukraine, a point beyond which Russian aggression toward the country would incite a dramatic American response.

Mr. Blinken wouldn’t bite. “The U.S. has real concerns about Russia’s unusual military activity on the border with Ukraine,” he said, with notable understatement. No red line was drawn. The State Department spokesman batted aside a similar question on Tuesday, saying only that “any escalatory or aggressive actions would be of great concern.”

U.S. officials often avoid questions about red lines, which when crossed can damage their credibility if they do not act. But in the case of Russia — which has been moving the estimated 90,000 troops it has on its border with Ukraine in ways that officials say might presage an invasion — the Biden administration has been conspicuously vague about when, and how, it might come to Ukraine’s defense.

That has raised questions about how far President Biden would be willing to go in a confrontation with Russia’s president, Vladimir V. Putin, over Ukraine. Mr. Biden’s Democratic predecessor, Barack Obama, resisted strongly increasing America’s commitment to Ukraine because, he said, Mr. Putin would always raise the ante even more. Mr. Obama otherwise largely delegated Ukraine affairs to Mr. Biden, then his vice president, who visited the country several times, extolling its independence.

Despite his personal investment in Ukraine’s fate, however, it is unclear how much Mr. Biden may be willing to risk on the country’s behalf as he fixates on competition with China. His senior officials have said repeatedly that their goal with Russia is to develop a relationship of stability and predictability.

In a statement on Wednesday honoring the millions of Ukrainians who starved to death under Joseph Stalin, Mr. Biden said the United States “also reaffirms our commitment to the people of Ukraine today and our unwavering support for the sovereignty and territorial integrity of Ukraine.”

For now, the Biden administration is focusing on diplomatic channels to deter and dissuade Mr. Putin, according to people familiar with the matter. Those could eventually include a second face-to-face meeting between Mr. Biden and Mr. Putin: Last week, the Kremlin’s spokesman said Washington and Moscow were discussing another potential summit.

Mr. Biden’s administration views continued dialogue with Mr. Putin as important to stopping him from taking action against Ukraine.

At the same time, U.S. officials are hammering out steps with partner countries to punish any Russian provocations, including new economic sanctions.

“We have administration officials in Europe trying to work towards coordinating what those economic measures would be,” said Andrea Kendall-Taylor, a Russia expert with the Center for a New American Security who advised the Biden transition team. She said such action could include sanctions on Russian banks and energy producers and the country’s sovereign debt.

Nevertheless, American and British officials have discussed imposing tougher sanctions on people close to Mr. Putin, including some measures that were considered, but put aside, after Moscow’s agents used a nerve agent in an attempt to kill a former Russian intelligence officer in Britain in 2018.

Measures under consideration include blocking Russian oligarchs from using Visa and Mastercard credit cards and restricting where they and their families can travel in Britain and Europe, as well as other kinds of sanctions that might get Mr. Putin’s attention quickly but damage parts of the American or European economy.

A Ukrainian official said the United States was considering a package of increased military aid to Ukraine. (The Biden administration is sending more than $400 million in security assistance to Ukraine this year.) But American officials are hesitant to discuss Washington’s support, for fear of escalating the situation further.

Fiona Hill, who served as the National Security Council’s director for Russia in the Trump White House, added that one of Mr. Putin’s goals was to strike a deal with the United States that excluded European input.

“This is really a challenge to Europeans to step up in solidarity with the United States,” she said. “The United States shouldn’t be the prime mover here.”

But Ms. Kendall-Taylor said Mr. Putin might doubt Western willingness to follow through. “I think there is a calculation on Putin’s part that there will be a lack of resolve in the West,” she said, adding that the Russian leader recognizes that the United States in particular is determined to focus its attention on China.

Russia has not explicitly threatened to invade Ukraine, but it has complained of alleged provocations from the Ukrainian side of their shared border. Mr. Putin has supported a pro-Russian separatist insurgency in the former Soviet republic’s east since 2014, when a popular revolution ousted Ukraine’s Putin-backed president. Russia annexed Ukraine’s Crimean Peninsula soon afterward.

In a sign of the increasing tensions, the chairman of the Joint Chiefs of Staff, Gen. Mark A. Milley, spoke by phone on Tuesday with his Russian counterpart, Gen. Valery Gerasimov. The Pentagon said in a statement that the call was meant to “ensure risk reduction and operational de-confliction.”

Ukraine’s president, Volodymyr Zelensky, has said that the United States is providing information and data to Kyiv about the Russian force buildup. Western officials have confirmed that NATO allies are stepping up intelligence sharing with Ukraine, in hopes that a better understanding of the rising threat will help Kyiv better prepare and better deter Moscow.

Even in worst-case scenarios, most analysts say, Kyiv should not expect the U.S. military to come to its rescue.

“The Russians know full well, because they’ve been invading Ukraine for seven years now, that we’re not going to send in the 82nd Airborne,” said Samuel Charap, a former State Department official now with the RAND Corporation. “And I think they have likely priced in everything short of that, in the sense that they are willing to pay the price.”

“That’s what makes this hard,” he added. “There’s no easy way out of this.”

American officials said they did not believe that Mr. Putin had yet decided whether to take military action against Ukraine. While the threat is being taken seriously, officials said, the United States and its allies have time to try to prepare Kyiv and convince Moscow that such a move would be a terrible mistake.

Whatever Mr. Putin’s thinking, his troop buildup is likely to test the willingness of the United States, NATO and Europe to act.

“The buildup of Russian forces is in part to see what Brussels is going to do and what Washington is going to do,” said Martijn Rasser, a former C.I.A. officer and now a senior fellow at the Center for a New American Security. “Putin sees one goal of this military action as gauging the resolve of the West when it comes to having Ukraine’s back.”