WASHINGTON — President Biden said on Wednesday that he now expected President Vladimir V. Putin of Russia would order an invasion of Ukraine, delivering a grim assessment that the diplomacy and threat of sanctions issued by the United States and its European allies were unlikely to stop the Russian leader from sending troops across the border.

“Do I think he’ll test the West, test the United States and NATO, as significantly as he can? Yes, I think he will,” Mr. Biden told reporters during a nearly two-hour news conference in the East Room of the White House. He added, almost with an air of fatalism: “But I think he will pay a serious and dear price for it that he doesn’t think now will cost him what it’s going to cost him. And I think he will regret having done it.”

Asked to clarify whether he was accepting that an invasion was coming, Mr. Biden said: “My guess is he will move in. He has to do something.”

The president later acknowledged that Mr. Putin’s move might not amount to a full-scale invasion of the country.

Still, Mr. Biden’s comment went well beyond the current intelligence assessments described by White House officials, which conclude that Mr. Putin has not made a decision about whether to invade. The comment is also likely to provoke concern in Ukraine and among NATO allies, because Mr. Biden acknowledged that if Mr. Putin conducted only a partial invasion, NATO nations could be split on how strongly to react.

“It’s very important that we keep everyone in NATO on the same page,” Mr. Biden said. “That’s what I’m spending a lot of time doing. There are differences. There are differences in NATO as to what countries are willing to do, depending on what happened, the degree to which they’re able to go.”

Pentagon officials say that such an invasion, intended to split and destabilize Ukraine, would most likely extend Moscow’s control of eastern regions of the country, where a grinding war with Russian-backed separatists has been underway in the eight years since Russia annexed Crimea.

But the president also seemed to contradict some of his own aides, who have said in the past week, in background briefings for reporters, that there would be no distinction between a small incursion into Russian-speaking territory in Ukraine and a full attack on the country. An invasion is an invasion is an invasion, one State Department official, speaking to reporters on the condition of anonymity, said last week.

A half-hour after the president ended his news conference, Jen Psaki, the White House press secretary, issued a clarification of his remarks, saying that Mr. Biden would treat any move over the border as an invasion — but was reserving judgment on how NATO would respond to other kinds of attacks.

“If any Russian military forces move across the Ukrainian border, that’s a renewed invasion, and it will be met with a swift, severe and united response from the United States and our allies,” she said in a statement. But she added that cyberattacks and paramilitary action might be treated differently, “with a decisive, reciprocal and united response.”

Republicans leapt on Mr. Biden’s description of a NATO that could be easily divided on how to react, depending on whether Russia conducts a full-scale invasion or a more subtle undermining of the Ukrainian government.

“President Biden’s remarks on Russia’s buildup near Ukraine tonight were nothing short of a disaster,” said Representative Michael McCaul of Texas, the top Republican on the House Foreign Affairs Committee, adding that the president “shared the potential disunity of Western nations on tough sanctions and clearly gave Vladimir Putin the green light to launch a ‘minor incursion.’”

The president’s comments came as Russia has marshaled roughly 100,000 troops, backed by tanks and heavy armor, on three sides of Ukraine. Mr. Biden has vowed to impose extensive sanctions if an invasion happens, but he acknowledged that responses could differ depending on the extent of the attack. For example, he noted that even crippling cyberattacks, of the kind Russia used to take out the power grids in parts of Ukraine in 2015 and 2016, might bring about a different response.

“It’s one thing to determine that if they continue to use cyberefforts, we will respond the same way, with cyber,” he said. But the president cut himself off, so it was unclear if he was suggesting that a cyberattack on Ukraine would result in a U.S.-led or NATO-led cyberretaliation against Russia. While the United States has quietly conducted tabletop war games to simulate such an exchange, there are concerns that it could quickly escalate and lead to more Russian cyberattacks on American targets.

The president appeared at one point to offer an off-ramp to the Russian leader, saying aloud what his negotiators have said in private to the Russians about Mr. Putin’s demands that Ukraine never be allowed into NATO and that the United States not base nuclear weapons there. Ukraine would not be accepted into the NATO alliance for years, Mr. Biden said. He added that he could assure Mr. Putin — as he did in a phone call several weeks ago — that the United States had no intention of basing nuclear weapons in there.

But when pressed, the president suggested there was no room to negotiate on Mr. Putin’s other demands: that all American and NATO troops be pulled out of countries that once were part of the Soviet bloc, and that all American nuclear weapons be removed from Europe. Both of those demands are included in a draft “treaty” that Mr. Putin’s government sent to the United States and NATO nations in December, demanding written answers — which so far have not been forthcoming.

“We’re going to actually increase troop presence in Poland and Romania, et cetera, if in fact he moves,” Mr. Biden said. “Because we have a sacred obligation” to defend those nations, both of which are NATO nations.

“We don’t have that obligation relative to Ukraine, although we have great concern about what happens in Ukraine,” he added.

Mr. Biden’s news conference came just 36 hours before Secretary of State Antony J. Blinken, one of Mr. Biden’s longest-serving national security advisers, was scheduled to meet his counterpart, Sergey V. Lavrov, in Geneva on Friday. But Mr. Biden said at one point that he was not sure whether the diplomats Mr. Putin had negotiating on his behalf understood what their own leader wanted — or what he would decide.

Mr. Biden portrayed Mr. Putin as more a tactical thinker than a strategic one, describing him as caught between larger, richer nations — and increasingly desperate to restore the kind of power the Soviet Union had when Mr. Putin was rising up as an intelligence officer in the K.G.B.

“I think that he is dealing with what I believe he thinks is the most tragic thing that’s happened to Mother Russia,” Mr. Biden said, “in that the Berlin Wall came down, the empire has been lost.”

“He is trying to find his place in the world between China and the West,” he said.

As to Mr. Putin’s decision about invading Ukraine, Mr. Biden added, “I suspect it matters which side of the bed he gets up on in the morning as to exactly what he’s going to do.”

Asked whether he still believed, as he said in Geneva in June after meeting with Mr. Putin, that “the last thing” the Russian leader wanted was a restoration of the Cold War, the president hesitated a moment.

“I still think he does not want any full-blown war,” he said, while not addressing whether Mr. Putin was interested in the kind of short-of-war actions that the two powers took against each other between the late 1940s and the dissolution of the Soviet Union in 1991.

When Mr. Biden took office a year ago, he said he wanted a “predictable and stable” relationship with Russia. His first act was to renew the New START nuclear treaty, which limits each country to 1,550 nuclear weapons. That was intended to avoid a renewed arms race between the two countries.

But Russia quickly acted in other arenas. It deployed troops near Ukraine in April, though not on the scale of the recent effort to surround the country. A series of ransomware attacks on American companies alarmed the Biden administration, especially after one, on Colonial Pipeline, disrupted the flow of gasoline, diesel and jet fuel up the East Coast. The provocations prompted the only face-to-face summit between Mr. Biden and Mr. Putin so far, though Mr. Biden said on Wednesday he was open to another one if he thought it would help defuse the Ukraine crisis.

Mr. Putin has argued that Russia has been increasingly surrounded by NATO forces, and that Ukraine’s shift toward the West is a major security threat to Moscow. So he has proposed essentially scrapping an agreement that President Bill Clinton and President Boris N. Yeltsin of Russia came to in 1997, which allowed former members of the Soviet bloc to decide for themselves whether they wanted to align with NATO, lean toward Russia or adopt some kind of neutral position.

If Mr. Putin is successful, he will have unwound the fundamental understandings of how Europe has been organized since the Soviet Union collapsed. But in answering questions on Wednesday afternoon, Mr. Biden suggested that the implications of a decision by Russia to invade Ukraine would reach much farther.

“If he invades, it hasn’t happened since World War II,” Mr. Biden said. “This will be the most consequential thing that’s happened in the world in terms of war and peace since World War II.”