Two Russian Tupolev Tu-95 strategic bombers took off from Russian airfields for a possible attack on Ukraine, the Ukrainian military warned in a post on Telegram early on Tuesday morning.
"We will provide additional updates if the situation changes," they added. Air raid warning signals should not be ignored, the military commented.
The military later claimed that "probable launches" of cruise missiles from Russian strategic bombers were detected.
Debris from destroyed Ukrainian drones sparked fires at an oil refinery and a hospital roof in Russia's Volgograd region, the administration of the southwestern Russian region said on Tuesday in a post to Telegram.
"Firefighters are working on site trying to contain and extinguish the fires," the administration cited the region's governor, Andrei Bocharov, as saying in its post.
US Chair of the Joint Chiefs General Dan Caine and his staff have been working on potential security guarantees for Ukraine, according to a source familiar with the planning, cited by CNN.
While the sources did not provide details to CNN on what possibilities for US involvement in the Russia-Ukraine war are being discussed, the CNN report noted that Trump has declined to rule out sending US troops to the region.
Ukraine offers $100b. weapons deal to obtain US security guarantees
Ukraine will promise to buy $100 billion of US weapons financed by Europe as part of a deal to get guarantees from the United States for its security after a peace settlement with Russia, the Financial Times reported on Monday, citing a document seen by the newspaper.
The newspaper added that under the proposals, Ukraine and the US would also strike a $50 billion deal to produce drones with Ukrainian companies.
This is a developing story.