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

Secretary of State Antony Blinken, changing plans at the last minute to speak at a U.N. Security Council meeting Thursday, implored the group to unite to address what he called "Russia's looming aggression against Ukraine."

"This crisis directly affects every member of this council and every country in the world because the basic principles that sustain peace and security principles that were enshrined in the wake of two World Wars, in a Cold War, are under threat: The principle that one country cannot change the borders of another by force. The principle that one country cannot dictate another's choices or policies, or with whom it will associate the principle of national sovereignty," he said.

Blinken repeated that the U.S. and Western allies do not see Russian troops drawing down on the ground, but that, "Our information indicates clearly that these forces, including ground troops, aircraft ships, are preparing to launch an attack against Ukraine in the coming days," he said.

He said that Russia is already taking steps "down the path to war," first with its plans to "manufacture a pretext for its attack."

"This could be a violent event that Russia will blame on Ukraine, or an outrageous accusation that Russia will level against the Ukrainian government. We don't know exactly what formula. It could be a fabricated so-called terrorist bombing inside Russia. The invented discovery of the mass grades. A staged drone strike against civilians, or a fake -- even a real attack -- using chemical weapons,” he said.

He said, then, the would see the "highest levels of the Russian government may theatrically convene emergency meetings to address the so-called crisis." 

"Next, the attack is planned to begin. Russian missiles and bombs will drop across Ukraine. Communications will be jammed. Cyber attacks will shut down key Ukrainian institutions. After that, Russian tanks and soldiers will advance on key targets that have already been identified and mapped out in detailed plans," he said, reiterating the U.S. belief that Ukraine's capital Kyiv, a city of 2.8 million people, is a target.

He added that the U.S. has intelligence "Russia will target specific groups of Ukrainians."

"Let me be clear," Blinken said, "I am here today not to start a war, but to prevent one."

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