KYIV, Ukraine — The United States and Russia continued to clash over the fate of Ukraine, with President Biden warning Thursday that the threat of a Russian attack remained “very high,” even as the Kremlin insisted that it was withdrawing troops from border areas and said in writing it was not planning an invasion.
But Russia repeated its threat of unspecified “military-technical measures” if the United States did not accede to its demands for sweeping changes to security arrangements in Eastern Europe, a signal that President Vladimir V. Putin would use his large-scale military buildup around Ukraine to extract concessions from Western nations determined to avert a war.
The Kremlin continued to keep the United States and its Western allies off-balance, sounding positive notes about diplomacy in a written response to U.S. security proposals, and offering the most detailed accounting so far of what Mr. Putin said on Tuesday was a “partial” pullback of the 150,000 troops that the United States estimates it has massed around Ukraine.
At the same time, Russia and Ukraine exchanged blame for a flare-up of shelling in Ukraine’s northeast that U.S. officials said they were “watching closely” out of concern that Russia could use it as a pretext to invade. And the State Department announced that Moscow had expelled the deputy American ambassador to Russia last week, calling it an “escalatory step” that would hinder diplomatic efforts.
Speaking at the United Nations Security Council, Secretary of State Antony J. Blinken said that despite Russian denials, U.S. intelligence believed Mr. Putin would launch an assault against Ukraine, and challenged Moscow to say it would not.
The dizzying back-and-forth on Thursday was punctuated by Mr. Biden, who said in brief remarks outside the White House that while “there is a path” to a diplomatic resolution, he still expected Mr. Putin to launch an invasion within several days.
“Every indication we have is they’re prepared to go into Ukraine,” he said.
Western officials continued to say they had seen no sign of a Russian pullback of the forces menacing its smaller neighbor. Secretary of Defense Lloyd J. Austin III, who met with the head of NATO in Brussels, said Russia continued to move troops closer to Ukraine’s borders, was adding combat aircraft and was stocking up on blood supplies in anticipation of casualties on the battlefield.
“I was a soldier myself not that long ago,” Mr. Austin said. “I know firsthand that you don’t do these sorts of things for no reason. And you certainly don’t do them if you’re getting ready to pack up and go home.”
The Russian Defense Ministry said on Thursday that troops had redeployed hundreds of miles away from the Ukrainian border areas after conducting military exercises.
The ministry’s spokesman, Maj. Gen. Igor Konashenkov, said that logistics units of the western military district had traveled more than 400 miles from the Kursk region, bordering Ukraine, and returned to their base in the town of Dzerzhinsk in central Russia.
Several other military groups traveled more than 900 miles by railway with their equipment and were redeployed to Chechnya and Dagestan, he said. The troops currently engaged in military exercises in Belarus, to Ukraine’s north, will also return to their home bases once the drills are over, General Konashenkov said in a statement.
The Kremlin spokesman, Dmitri S. Peskov, insisted the withdrawal was continuing. “This process takes time,” he said. “They cannot just get lifted in the air and fly away.”
Since late fall, the number of troops that Russia has dispatched to the borders near Ukraine has been growing. In the first week of January, the United States estimated that they numbered around 100,000. That figure grew to 130,000, and then, on Tuesday, Mr. Biden put the number at 150,000 — with brigades normally based as far away as Siberia joining the force.
On Wednesday, a senior American official, who refused to be quoted by name, told reporters that far from winding down its deployment, Moscow was adding 7,000 combatants.
Russia has framed the crisis as revolving around its fundamental security. And it says that even the distant prospect of Ukraine joining NATO represents an existential threat.
President Volodymyr Zelensky of Ukraine made it clear once again that NATO membership is key to his country’s long-term security. “It’s not an ambition,” he said in brief comments to the BBC. “It’s our life.”