"My sense is this will happen in the next several days," he told ABC News Chief White House Correspondent Cecilia Vega as he left the White House for a trip to Ohio.
Biden told reporters the threat of an invasion is "very high."
"They have not moved any of their troops out. They have moved more troops in," he said. "We have reason to believe that they are engaged in a false flag operation, to have an excuse to go in. Every indication we have is they're prepared to go into Ukraine and attack Ukraine."
A senior administration official said Wednesday evening the Kremlin had added about 7,000 troops near the Ukraine border in recent days -- "with some arriving as recently as today" -- placing the number of Russian forces near Ukraine above the 150,000 figure cited by Biden in an address to the nation Tuesday.
At the same time, Biden repeated there is still a path to diplomacy but said he has no plans to hold a call with Putin.
"That's why I asked Secretary Blinken to go to the U.N. to make his statement today," he said, referring to Secretary of State Antony Blinken changing plans at the last minute to address the U.N. Security Council. "He'll lay out what that path is. I've laid out a path to Putin as well ... There is a path, there is a way through this."
Even as Russia delivered its written response Thursday to U.S. and NATO proposals for talks, a senior administration official dismissed Russia's diplomacy, so far, as disingenuous, to reporters on an earlier call.
"Every indication we have now is that they mean only to pretend to engage in diplomacy, where they publicly offer to talk and make claims about de-escalation -- while privately mobilizing for war," the official said.
For days now, the Kremlin has claimed to be pulling back Russian forces after completing military "exercises," but U.S. security analysts have said it appears Russian troops are rotating in and growing in number, with the ability to invade Ukraine at any point. Russian leaders, meanwhile, have accused the West of creating "hysteria" over the situation at Ukraine's border.
The buildup of Russian forces has prompted the biggest concentration of troops in Europe since the Cold War, the NATO secretary-general said Wednesday.
Biden warned Putin in remarks from the White House earlier this week that if Russia invades Ukraine, the U.S. is prepared to respond decisively and in unison with NATO allies.
"If Russia attacks Ukraine, it will be met with overwhelming international condemnation," Biden said. "The world will not forget that Russia chose needless death and destruction. Invading Ukraine will prove to be a self-inflicted wound. The United States and our allies and partners will respond decisively. The West is united and galvanized."
Explaining U.S. involvement in the region, Biden told the American people, "this is about more than just Russia and Ukraine."
"It's about standing for what we believe in, for the future that we want for our world, for liberty, for liberty, the right of countless countries to choose their own destiny. And the right of people to determine their own futures, or the principle that a country can't change its neighbor's borders by force," Biden said. "If we do not stand for freedom where it is at risk today, we'll surely pay a steeper price tomorrow."