Belarus' Defence Ministry said Russia and Belarus would extend joint military exercises that had been due to end on Sunday, dashing Western hopes that Russia would swiftly withdraw forces that could be used in a potential invasion of Ukraine.
The ministry did not say how long Russian troops in Belarus - estimated by NATO to number 30,000 - might now remain in the country, which lies north of Ukraine.
The decision to extend was taken because of increased military activity near the borders of Russia and Belarus and an escalation of the situation in the eastern Ukrainian region of Donbass, the Belarus ministry said in a statement.
The next stage would address defence areas that had not been fully covered by the earlier stages of training, Belarus Defence Minister Viktor Khrenin said.
"In general, its focus will remain unchanged - it is designed to ensure an adequate response and de-escalation of military preparations of ill-wishers near our common borders," Khrenin said.
There was no immediate comment from Russia.
Russia's build-up of forces surrounding Ukraine, currently estimated by the West at more than 150,000 to the north, east and south, has prompted NATO to dispatch reinforcements to eastern Europe.
Moscow, which denies preparing to invade Ukraine, has seized on this to argue that it is the United States and its allies who are whipping up tensions.
Demonstrating the close alliance between Moscow and Minsk, Belarus' President Alexander Lukashenko sat with Vladimir Putin in a "situation centre" on Saturday while the Russian president observed strategic nuclear exercises involving the launch of hypersonic and cruise missiles.
(Reporting by Polina Devitt; Writing by Mark Trevelyan; Editing by Frances Kerry)
