The United States has received a response from Russia regarding the discussion over Moscow's demands for security guarantees that NATO not expand, a senior official with the U.S. Department of State said Thursday.

"We can confirm that we have received a response from the Russian Federation," the official said. "It was delivered to Ambassador Sullivan in Moscow today."

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.

Russian Foreign Minister Sergey Lavrov said earlier Thursday that Moscow was planning to send its formal response to the Washington soon.

-ABC News' Conor Finnegan

U.S. Ambassador to the United Nations Linda Thomas-Greenfield warned Thursday that Russia appears to be readying for an "imminent invasion" of neighboring Ukraine.

"The evidence on the ground is that Russia is moving toward an imminent invasion," Thomas-Greenfield told reporters during a background briefing call. "This is a crucial moment."

"Our goal is to convey the gravity of the situation," she added. "That’s why Secretary Blinken is coming to New York to signal our intense commitment to diplomacy, to offer and emphasize the path toward de-escalation and to make it clear to the world that we are doing everything -- everything -– we can to prevent a war."

Blinken has already boarded his flight to Munich but will be flying to New York City first to address the U.N. Security Council session at 10 a.m. ET on Thursday.

-ABC News' Conor Finnegan

Belarusian President Alexander Lukashenko said Thursday that Russian troops will be in his country for "as long as necessary."

"As long as necessary, the Russian Armed Forces will be here. This is our land, our territory," Lukashenko told Belarusian state media. "We are working here, studying and will continue to study."

Further joint exercises of the armed forces of Russia and Belarus will be discussed with Russian President Vladimir Putin on Friday, according to Lukashenko.

The Belarusian leader has said that any decision on recognizing the self-declared Donetsk and Lugansk people's republics in a breakaway region of southeastern Ukraine known as Donbas will be made jointly with Russia. He added that this decision will be mutually beneficial.

Lukashenko has also noted that, in his view, if neighboring Ukraine had wanted to end the conflict with the Russian-backed separatists in Donbas, it would have already done so.

-ABC News' Tanya Stukalova

Russia will soon formally respond to the written answers that the United States sent earlier to Moscow's demands for security guarantees that NATO not enlarge, Russian Foreign Minister Sergey Lavrov said Thursday

The U.S. government, at Russia's insistence, sent written responses to two draft treaties the Kremlin 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.

Lavrov was quoted by Russian state media on Thursday as saying that Moscow is planning to send its formal response "today" and that it will be published "several hours after."

However, Russian Deputy Foreign Minister Alexander Grushko told reporters Moscow was "still working on" its response and that the reply would not be sent Thursday.

Russian President Vladimir Putin has said his government will continue to insist on discussing its key demands alongside any of the other issues.

-ABC News' Patrick Reevell and Tanya Stukalova