Belarus's defence ministry said on Wednesday that joint Russian-Belarusian air defence forces had been reinforced, with new missile units moved into position.
"Anti-aircraft missile units advanced to designated areas and took up combat duty," the ministry said in a brief statement.
Belarus is a close ally of Russia and allowed President Vladimir Putin to use its territory as one of the launchpads for his Feb. 24 invasion of Ukraine. The two countries plan to hold joint aviation drills in the second half of January, beginning next Monday.
Ukraine has repeatedly warned that Putin may try to use Belarus to launch a new invasion of Ukraine from the north, a step that would open a major new front in the war.
(Reporting by Reuters; Editing by Gareth Jones)