Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty in speech denouncing West

ABC News has obtained a letter sent by a top Ukrainian official to senior members of Congress, asking them to assist Ukraine's calls for additional air defense systems to counter the attack drones built and supplied by Iran to bolster Russia's war effort.

Russia has launched waves of deadly attacks in recent weeks, using Iranian-made drones that explode on impact to strike power plants, killing civilians and causing rolling blackouts, plunging millions of Ukrainian homes into darkness.

In the letter, the chairman of the Ukrainian parliament, Ruslan Stefanchuk, calls on the U.S. to provide Ukraine with highly mobile air defense systems known as C-RAMs, saying they would help protect "important objects, especially crucial power plants."

-ABC News' Tom Soufi Burridge, Luis Martinez and Natalya Kushnir

U.S. Ambassador to the United Nations Linda Thomas-Greenfield traveled to Ukraine on Tuesday "to reiterate the United States' unwavering support as Ukraine defends its freedom and territorial integrity amidst Russia's brutal and unprovoked invasion," according to a statement from the U.S. Mission to the U.N.

While in the country's capital, Kyiv, Thomas-Greenfield "will meet with Ukrainian government leaders to discuss the ironclad American commitment to the sovereignty and independence of Ukraine," and she "will observe efforts to document and preserve evidence of atrocities committed by Russian forces and will hear first-hand accounts of survivors," according to the U.S. mission.

"Ambassador Thomas-Greenfield will also discuss the global food insecurity crisis exacerbated by Russia's invasion and will underscore the critical need for an extension of the Black Sea Grain Initiative into the coming year," the U.S. mission said. "She will also meet with humanitarian organizations working to meet winterization needs for vulnerable people impacted by Russia's attacks on energy and other civilian infrastructure."

President Joe Biden spoke to German Chancellor Olaf Scholz on Sunday and both agreed Russia's nuclear threats stemming the war in Ukraine are "irresponsible," according to the White House.

Both leaders said they would continue to "provide Ukraine with the economic, humanitarian, and security support it needs to defend against Russia," the White House said in a statement.

Biden and Scholz also spoke of the chancellor's recent trip to the People's Republic of China and, according to White House officials "affirmed their shared commitment to upholding the rules-based international order, human rights, and fair trade practices.”

-ABC News' Justin Gomez

The U.S. announced a new $400 million aid package for Ukraine that will help Kyiv procure tanks.

The aid money will go toward refurbishing 45 Soviet-era T-72 tanks belonging to the Czech Republic's defense industry, according to Pentagon spokesperson Sabrina Singh.

The Netherlands has also committed to paying for another 45 tanks to be refurbished for Ukraine.

The first of a total of 90 tanks is set to arrive before the end of the year, according to Singh.

"Since 2014, the US has committed more than $21 billion in security assistance and more than $18.2 billion since the beginning of Russia's unprovoked brutal invasion on Feb. 24," Singh said.

-ABC News' Matt Seyler