Belarus on Saturday accused a Ukrainian military helicopter of flouting its border during maneuvers and flying one kilometer (0.62 miles) into Belarusian territory.
The state border service issued a statement and a video of what it said was the Ukrainian helicopter flying across a snowy, wooded area at a checkpoint. A spokesman for Ukraine's state border guard service said the allegation was false.
The accusation comes amid wider tensions in the region, with Ukraine and its NATO allies accusing Russia of massing tens of thousands of troops near Ukraine's borders in what they say might be preparation for an attack. Russia denies any such plan.
"The Ukrainian Mi-8 military helicopter at an altitude of about 100 meters crossed the state border and flew to a depth of 1 km into the territory of Belarus. The Ukrainian side was informed about this incident," said the Belarusian statement.
A spokesperson for Ukraine's border guard service denied the accusation.
"As part of strengthening the protection of the border with Belarus, aviation is used to monitor the border, but there have been no violations of the state border by aviation," spokesperson Andriy Demchenko said.
Relations between Belarus and Ukraine have plummeted since last year, when Russia sided with Belarusian President Alexander Lukashenko during mass street protests and Lukashenko in turn became more vocal in his support for Moscow against Ukraine.
Ukraine has accused Belarus and Russia of manufacturing a migrant crisis on the EU's eastern borders with Belarus that has left thousands of people stranded in freezing conditions.
Lukashenko denies orchestrating the migrant crisis and his government promised an "asymmetrical" response against a new round of sanctions imposed on Minsk on Thursday.
He also signaled that Belarus could side with Russia in a military conflict with Ukraine and that he recognized Crimea - the territory annexed by Russia from Ukraine in 2014 - as Russian.