Russia-Ukraine updates: US to ban Russian carriers from its airspace

In a written response on the discussion over Moscow's demands that NATO not expand, Russia said the U.S. has not given a "constructive reply" to Russia's demands and that Moscow might resort to "military-technical measures."

"We can acknowledge that the U.S. side did not give a constructive answer to the basic elements of a draft treaty with the U.S. on security guarantees, drawn up by the Russian side," the document given to the U.S. on Thursday and translated from Russian by Interfax said. "In the absence of the American side's willingness to negotiate firm and legally binding guarantees of our security by the U.S. and its allies, Russia will have to respond, including through implementing military-technical measures," it said.

As the West has warned of Russian misinformation serving as potential for a false flag pretext, the document from the Russian Federation accused the U.S. and allies of spin, instead.

"The package nature of Russian proposals has been ignored, from which 'convenient' topics have been deliberately chosen, which, in turn, are 'twisted' in the direction of creating advantages for the United States and its allies," the response said. 

"We propose to work together to develop a new 'security equation'," it added.

The U.S. government, at Russia's insistence, sent written responses to two draft treaties Moscow published demanding guarantees that Ukraine will never join NATO and that the military alliance pull back its infrastructure from Eastern European countries that joined after the Cold War. Washington rejected those guarantees as non-starters but offered to discuss some confidence-building measures.

-ABC News' Tanya Stukalova

Ukrainian President Volodymyr Zelenskyy on Thursday visited Ukrainian troops on the front line of defense in the Donetsk region, where the Ukrainian government says intense shelling continues.

"I am proud of your heroism," Zelenskyy told the soldiers. "You defend Ukraine's independence and decently rebuff the enemy. We feel constant pride in our defenders."

Earlier Thursday, Zelenskyy had an "urgent" telephone conversation with the head of the European Council, during which he emphasized that Ukraine has seen no evidence of withdrawal of Russian troops from its borders, according to a press release from the Ukrainian president's office.

Zelenskyy also reported a "significant number" of ceasefire violations by Russian-backed separatists forces in eastern Ukraine, including the shelling of a kindergarten in the government-controlled village of Stanytsia Luhansk, which he called a "large provocation," according to the press release.

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