Russia and Belarus have begun joint military exercises which have triggered fears that Moscow could use its ally to launch a new ground offensive in Ukraine.
Russia used its neighbour Belarus as a springboard for its invasion of Ukraine last February.
The two allies will conduct air force drills from January 16 to February 1 using all Belarus military airfields and began joint army exercises involving a "mechanised brigade subdivision" on Monday, the Belarusian defence ministry said.
Minsk says the air drills are defensive and it will not enter the war.
"We're maintaining restraint and patience, keeping our gunpowder dry," said Pavel Muraveyko, first deputy state secretary of Belarusian Security Council, according to a post on the Belarusian defence ministry's Telegram app on Sunday.
Muraveyko said the situation on the country's southern border with Ukraine was "not very calm" and that Ukraine has been "provoking" Belarus.
"We are ready for any provocative actions on the part of Ukraine," he said.
Moscow denies it has been pressuring Belarusian President Alexander Lukashenko to take a more active role in the conflict in Ukraine.
Ukraine has continuously warned of possible attacks from Belarus and President Volodymyr Zelenskiy said last week the country must be ready at its border with Belarus.
Belarus has conducted numerous military exercises since the invasion began, both on its own and jointly with Russia. Together with Moscow, Minsk has also been bolstering the drills with weaponry and military equipment.
Unofficial Telegram military monitoring channels have been reporting a series of fighters, helicopters and military transport planes coming to Belarus since the start of the year - eight fighters and four cargo planes on Sunday alone.
Reuters was not able to verify the reports. The Belarusian defence ministry said only that "units" of Russia's air forces have been arriving in Belarus.
Ukraine saw little hope of pulling any more survivors from the rubble of an apartment block in the city of Dnipro on Sunday, a day after the building was hit during a major Russian missile attack, with dozens of people expected to have died.
Valentyn Reznichenko, governor of the Dnipropetrovsk region, said on Monday that 35 people were confirmed dead so far and the fate of 35 more residents remained unknown.
"The search for people underneath the rubble continues," Reznichenko said on Telegram.
Ukraine's Air Force said the apartment block was struck by a Russian Kh-22 missile, which is known to be inaccurate and that Ukraine lacks the air defences to shoot down. The Soviet-era missile was developed during the Cold War to destroy warships.
Moscow has been pounding Ukraine's energy infrastructure with missiles and drones since October, causing sweeping blackouts and disruptions to central heating and running water.
In his nightly address after the Dnipro strike, Zelenskiy called on Western allies to supply more weapons to end "Russian terror" and attacks on civilian targets.