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

U.S. ambassador to the United Nations Linda Thomas-Greenfield met with Ukrainian President Volodymyr Zelenskyy and members of his leadership team in Kyiv on Tuesday "to discuss the unwavering U.S. commitment to the sovereignty and territorial integrity of Ukraine," according to a statement from U.S. Mission to the U.N. spokesperson Nate Evans.

"She reiterated that the United States is steadfast in its support for Ukraine and is prepared to stand with Ukraine as long as it takes," Evans said.

"Ambassador Thomas-Greenfield and President Zelenskyy discussed international efforts to minimize the impact of Russia’s aggression on global food security, including through sustaining and expanding the UN-brokered Black Sea Grain Initiative, and to ensure accountability for war crimes and atrocities perpetrated on the Ukrainian people," he added. "She committed to continuing to work at the United Nations to strengthen international support for Ukraine's sovereignty and to urge Member States to defend international law and the Charter of the United Nations."

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