The Russian mission to the United Nations circulated a 49-page letter Wednesday night that it said included an investigation of atrocities committed by the Ukrainian government in its eastern provinces.

The letter alarmed U.S. officials, who thought it may be part of the false pretext for a Russian invasion that they've been publicly warning about for weeks.

"It's hard to draw any conclusion other than Russia plans to use today's U.N. Security Council meeting as part of an attempt to establish a pretext for a potential invasion, building upon disinformation and incendiary statements we've seen over recent weeks," a senior administration official told reporters Thursday morning.

The letter, obtained by ABC News, includes a document labeled "Joint Project of the Investigative Committee of the Russian Federation and RT News Channel," which alleges that Ukraine committed war crimes over the last eight years in the Donbas.

"Each of these allegations are categorically false," the senior administration official said. But they've seen a marked increase in them in the past few days and overnight, they added.

But the official stopped short of saying that Russia has fully mobilized to create a false pretext or that an invasion would follow shortly: "The kinds of reports that we're hearing, the kinds of claims that we're seeing made in Russian media are exactly the kinds of claims that we have indicated could form the basis of a pretext for a further invasion of Ukraine," they told reporters.

-ABC News' Conor FInnegan

Russia has expelled the second-most senior U.S. official at the American embassy in Moscow, a spokesperson for the U.S. Department of State said Thursday.

U.S. Deputy Chief of Mission Bart Gorman had not completed his tour and still had a valid Russian visa, according to the spokesperson, but Russia has ordered him to leave the country. The decision was "unprovoked, and we consider this an escalatory step and are considering our response," the spokesperson told ABC News.

The United States and Russia have been engaged in a diplomatic tit-for-tat for years, slowly shrinking the other's diplomatic mission in their respective countries by closing consulates, expelling diplomats or restricting visas -- with each side claiming the other took new action that prompted a response.

The U.S. mission in Russia is now running on a barebones staff, struggling to keep the embassy functioning. That's especially after Russia barred its citizens from working for the U.S. mission, which led the U.S. to shuttering all of its consulates in the country.

But the timing of Gorman's expulsion is notable, with tensions sky-high over a potential Russian invasion of Ukraine.

"We call on Russia to end its baseless expulsions of U.S. diplomats and staff and to work productively to rebuild our missions," the spokesperson said. "Now more than ever, it is critical that our countries have the necessary diplomatic personnel in place to facilitate communication between our governments."

-ABC News' Conor Finnegan

President Joe Biden told ABC News Chief White House Correspondent Cecilia Vega on Thursday morning that he believes Russian President Vladimir Putin will go through with an invasion of Ukraine within days.

Asked when departing the White House if it's his sense that an invasion would happen, Biden said, "My sense is this will happen in the next several days."

The president accused Russia of engaging in a "false flag operation" and said that despite the claims of pulling troops back, the Kremlin has actually moved more troops closer to Ukraine's border.

"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."

Although he said he has not yet read Putin's written response to the U.S. and that he has no plans for a call with the Russian leader, Biden repeated that there is still a path open to diplomacy.

Russia insisted again Thursday that some troops massed near Ukraine are returning to base, far from the border.

Russian Ministry of Defense spokesman Igor Konashenkov said in a statement that units from the Western and Southern Military Districts, which were stationed in western Russia and Crimea, are now moving back to their permanent bases. He said Russian troops taking part in drills in Belarus will also return to base once they finish the exercises this weekend.

Konashenkov noted that the first units have already arrived in Nizhny Novgorod, Chechnya and Dagestan, hundreds of miles from the border with Ukraine.

-ABC News' Patrick Reevell

In response to a plea on Twitter from a Ukrainian official, SpaceX CEO Elon Musk said Saturday that his high-speed internet service Starlink is now active in Ukraine.

"More terminals en route," he tweeted in a reply to Mykhailo Fedorov, Ukraine's vice prime minister and minister of digital transformation.

Earlier Saturday, Fedorov appealed directly to Musk and asked him to provide Ukraine with Starlink stations.

The terminals are small, portable satellite dishes on Earth that connect directly to Starlink satellites in space -- providing high-speed internet to rural and hard-to-reach locations. This is especially important for areas that have already lost access and could potentially help them avoid cyberattacks.

-ABC News' Gio Benitez